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THEBE is probably no subject connected with tbe war which, is more interesting than the power possessed by Paris of standing a siege. Wo have, therefore, thought it well to place before our readers the accompanying map of Paris, showing its fortifications. The drawing has been prepared from the latest maps which can be obtained, and is as accurate as is consistent with the scale.

Paris is situated, to nso a military term, > eheval on the Seine, near the junction of this river with the Mnrae, the Utter river covering the city on the east, while a bend of the former protects the western side. The fortifications consist, first, of a chain of detached forts or fortresses commanding the rivers, and, secondly, by an enceinte continue running right round the city. The bastion system of Vauban is adopted throughout.

The exterior lino of defence commences at St. Denis on the north, and extends to the Marnc at Nogent, a distance of about eight miles. It consists of nine fortresses, and opposes the direct advance of troops from Germany. On the south Paris is covered by a line of six forts, reaching from the Marne to the Seine, a distance of about seven miles. These complete the defence of the south; four are pentagonal and two quadrilateral. They are built of substantial materials, and contain bomb-proof barracks, and have oasemated curtains. The third exterier line of defence is eight miles long. It does not consist of a chain of forts, but of the great citadel, Mont Valerien, which

serves as a tete-du-pont to the bend of the Seine, which covers Paris to the east. Mont Valerian is supported on one side by Fort d'Issy, and on the other by St. Denis. It is the most important work of the whole. The front toward the river is 460 yards long, and the other four faces average 328 yards euch. There is bombproof accommodation for stores and 4,500 men. The ring of detached forts, all of which will be easily found in our map, extends over 22| miles.

Within this is seen the enceinte continue, or continuous line of fortifications. This consists of no fewer than ninety-four bastions. The exterior sides average 328 yards pjr bastion, forming an enceinte of 30,840 yards, or nearly seventeen miles, probably the largest work of the kind ever executed. The cost was enormous—to judge from Marshal Marmont's opinion, was well spent. Marmont considered the fortifications of Paris "an event the most useful and important in the defence of France." The escarp of the enceinte is 33ft. high, the ditch being 82ft. wide. The distance from the boulevard to the enceinte continue averages about 1,093 yards, and that from the enceinte to the forts is about the some. So long as the forts are unconquered it will be impossible to bombard Paris from a less distance than about 7,000 yards, and evon then the siege train will be under fire at a range rather too close to be pleasant.

In addition to these defences it is stated that earthworks are being raised from fort to fort, but we learn from private letters that these are of a very insignificant description, affording good cover, it is true, for riflemen, but not safe against even impounder guns. It is stated that more than 1,000 guns can be placed in position; but this estimate, no doubt, includes a very large number of weapons which, however effective at close quarters, will be practically useless at long ranges.

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THE DEFENCES OF PARIS.

THE FORECASTING OF STORMS.'
(Concluded from page 538.)

1)ERHAPS the most terrible storm of this character that ever occurred in the United States was that which destroyed the villages of Comanche (Iowa) and Albany (Illinois) situated upon opposite sides of the Mississippi, on the evening of June 3rd, 1860. This fearful storm will long be remembered by all persons who resided at the time of its occurrence anywhere in the region through which it passed. It commenced as two separate and distinct tornadoes, which, moving eastwardly in well-defined and nearly parallel courses, crossed Cedar River in Iowa 12 miles apart, and then gradually converged, until, at a point about 25 miles west of the Mississippi, they united, and thence advanced in a single column with indescribable fury. After crossing the Mississippi, the force of the storm gradually diminished, and it had subsided into a mild gule by the time it reached Lake Michigan. In this tornado one hundred and thirty-four lives were reported to have been lost west of the Mississippi alone, and over two thousand persons were by its ravages rendered homeless. A gentleman who witnessed the storm at the point of its greatest intensity, describes it as looking, when first observed, " merely like a threatening cloud; but it soon assumed the appearance of a huge serpent, extending from the clouds to the earth, and twisting and writhing with an undulating] motion, accompanied by a roaring more terrible than that of the mightiest cataract."

Tornadoes like these, though generally somewhat less intense and destructive than these, prevail every season, and rage with no less fury upon the lakes than upon the land ; and as they move almost uniformly from west to east, or from south

• Extracted from an article by L. A. Roberts In the Western Monthly.

west to north-east, and at a range of speed riM1 generally from 20 to 40 miles an hour, that »*• seem to be no reason why such a system oi *rreporting as that just established by the G**31 in. ut should not be made immediatel'v aviiU-- rendering invaluable service to our laic cos.'•-"• As to the entire practicability of the scheox '-■ can be no question. General Myer, in a la"1 General Paine upon the Bubject, quotes frei' telegraphic news despatches in a single u»J' Washington newspaper a series of report* S* different points in the west and south-west * together map out clearly the coarse and * progress of a storm of wind and rain tliat pfthroughout a wide extent of territory in hr last. These reports were made without C cert or system, of course, but showcouclojrt' valuable practical results may be easilv P* through intelligent concert and system, tf*1 the different despatches.

"st. Louis, January 17.—A terrible S6&^ thunder and lightning, wind and hail, pa.ss.-j the city last evening."

11 Chicago, January 17.—During tbe tbHfc^ storm last night the mercury stood at 42° " («<

"louisville, January 17.—A terrible torn** visited Cave City Station, on the Louisville «" Nashville Railroad at an early hour this &>«"ing."

"Cincinnati, January 17.—An unusually b«v storm of wind and hail, accompanied with thm der and lightning, occurred here this morning.

u Pittsbuuoh, January 17.—A heavy rain'stom with thunder and lightning, visitodthis city at Bo" to-day."

There is no difficulty in deducing from these di! the fact that on the evening of January 16th, storm prevailed at St. Louis, which, moving f* wardly over a broad belt of territory, reached Chi cago some time during the same night, Cave Cit Station the next morning, Cincinnati later in t:

.morning, and Pittsburgh at noon of tho 17th,— uiviug traversed the distance from St. Louis to Pittsburgh in tome 18 hours, or at the rato of about W> miles' per hour. Prom the newspapers of the following day it might have been quite as easy to ^race the storm on to the sea-coast, and perhaps to tatlier particulars of the damage to shipping which t caused.

For several years the Smithsonian Institution ias been collecting meteorological observations from Jill parts of the country, and labouring to deduce t-1 mi the facts thus gathered the laws governing -lie phenomena of the weather and the climate ; and -lie valuable results so gained can now be drawn spoil in inaugurating the new system of storm prediction and reporting. "The determination of the Cull details of this system will be arrived at," says Oeiieral Myer, in a communication to the Secretary ~>t War, " only after careful study of the modes already tested in other countries, and consultation with experienced observers, telegraph companies, Ijoards of commerce, and business men, as to their application or improvement in our own. There will need to be the study aud determination of the points Cor observation ; the supply of instruments, aud the facilities for their use ; the exact observations to be :mriade; the exact form and times in which, when made, they are to be reported; the points at which xeports are to be collected and deductions from -them had; the places at which and the modes by which these deductions shall finally be announced, by telegraph ami signal, and so made useful to the public, for the benefit of commerce, by the warning they may give or the aids they may offer. It is a wise provision of the act,"he adds," that it enables the army to be thus extensively utilized in the interest of commerce, by the exercise of duties Already established, and which will require but little additional outlay. It would be needless and unwise to enter|upon large expenditures by attempting at the beginning too extended a scope for the endeavour. . . . I would suggest, therefore, that action under the resolution be limited, until the best modes for its execution shall have been wisely determined."

In accordance with this view, he asks for an appropriation, for carrying out the law, of the modest sum of 15,000 dollars, for the current fiscal year, ending June 30, and '.25,000 dollars, for the next fiscal year, ending June 30, 1871. As showing that the scheme is regarded with prompt favour by the interests it is designed to benefit, General Myer mentions the fact that the favourable proceedings upon the subject of six boards of commerce had, at the date of his communication, and within little more than a week after the approval of the joint resolution, been received at the office of the chief signal officer.

Professor Joseph Henry, of the Smithsonian Institution, writing to General Paine in approval of the objects of his joint resolution, points out the following conditions as essential to the success of the proposed system:

"1st. The points from which the telegrams are to be sent must be carefully selected and furnished with reliable instruments. 2nd. These instruments must be in charge of persons properly trained to moke the observations. 3rd. The telegrams must be transmitted regularly to some central point at fixed hours of the day. 1th. They must at tlus centre be collated and their indications interpreted by persons having a competent knowledge of the laws to which the motions of the storms are subjected. 5th. I do not think the military posts as now established will bo sufficient to fully carry out the plan; additional stations would be required. 6th. An appropriation would be necessary for the pay of the telegrams, furnishing the instruments, and the necessary superintendence."

And Professor Elias Loomis, of Yale College, author of a valuable text-book upon Meteorology, writes to General Paine upon the same subject at length. He says :—

"It is believed that our knowledge of storms is already sufficiently precise to enable a competent meteorologist to furnish information which would bj of great value to commerce, provided he had at his command a sufficient corps of observers scattered over a considerable area to the west and southwest, and also had the means of transmitting'.his warnings immediately by telegraph; and if such a system were pursued for several years, it could scarcely foil to conduce to more precise knowledge, which would render it possible to give more reliable and definite warning of the approach of dangerous storms.

"In order to secure the objects here contemplated, it would be indispensable to have observations from a pretty large number of stations, at intei vals not exceeding one »r two hundred miles, and scattered over a region to the west and southwest of those points for which the warnings were regarded as specially important. These observations should include all the usual meteorological instruments, but more particularly the barometer, with the direction and force of the wind. The observations should l>e made daily at fixed hours, and should be reported by telegraph to some competent meteorologist, whose business it should be to compare the reports without delay, and make the proper

deductions from them; and whenever a violent storm was in progress, to decide in what direction and with what velocity it was travelling ; determine what places it would visit, and at what hour it would arrive: and finally transmit the announcement immediately by telegraph to those places especially interested. Such a system could not be expected to attain satisfactory results without a pretty large number of well-selected stations, and especially without the service of a competent meteorologist L. superintend the entire system. The superintendent should be well informed respecting the progress which has been already made in this department of science; he should have strong faith in the practicability of attaining useful results by a system of storm-wamings; and he should have no other engagements which would prevent him from giving his whole attention to this subject, especially whenever a violent storm was raging in any part of the United States."

These coincident views of the two highest authorities in the United States upon all matters relating to meteorological science doubtless foreshadow substantially the actual working of the system when it shall have been fully established. In due time we may reasonably expect to see every lighthouse and other prominent and sightly point upon the borders of the great lakes and the Atlantic coast connected by telegraph with a central meteorological bureau, to which intelligence of an approaching storm can be simultaneously and speedily conveyed—to be in turn communicated, by means of a uniform system of signals, to all vessels within reach, in ample t<me to enable them to prepare for the coming danger.

That the results of the operation of this system will bo in the highest degree valuable, both as regards the immediate practical object of protecting our commerce from disaster, and as furnishing an aid to scientific investigation in a most important direction, there can be no reasonable doubt.

MUSSEL CLIMBING.

C1AN any one see a snail travel, and not ask J mentally, "how it does it?" The method certainly is curious. A fleshy disc is protruded, and caused to project in the direction of locomotion; it is then spread out flatly, and while slightly adhering to the object over which it is passing, a contractile energy is exerted, and the little animal bearing its house is drawn onward. Thus by the repeated protrusion, expansion, and contraction of this soft organ, in due time its journey is accomplished. Because of this method of progression on a ventral disc, all those shell-fish, or, properly speaking, molluscan animals, so constituted, are called by the systematists, gasteropoda, a term which means ventral-footed. And in rank these gasteropoda stand

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next to the most highly organized of the mollusca. But some of these shell-encased creatures do not travel at all. Take, for instance, the oyster, called a monomyary, because the valves are held together by a single muscle. This sedate bivalve once settled, probably never moves from that spot. But all the dimyaries, »r two-muscled bivalves, well represented by the common edible mussels, possess a foot, which is not greatly unlike that of the snails. The mussel's foot, however, presents, in its class, the least developed condition of this organ, for it is a spinner rather than a walker; or, as Owen says, "it is subservient to the function of a gland, which secretes a glutinous material analogous to silk, the filaments of which are termed the byssus," which often serves for attachment to rocks. He farther says, "in most dimyary bivalves the foot is an organ of locomotion." Some of the river mussels in babyhood spin a byssus with which to moor themselves against the currents of the stream. When older grown this necessity is overcome, and the capacity just mentioned is lost. Then the adult turns its foot into a ploughshare, and is dragged along in the farrow it makes in the mud. The razor-shell alternately bores downwards and propels upward, the foot doing all the work. With tho foot as an elastic spring the heart-shell leaps along. But the common black mussel, MiUjlus ethilia, and its despised neighbour, the brown horse-mussel, Modiola plieutula, who ever saw them walk? Propulsion is not always walking. The scallop with its large

• By Rev. S. Locewood, Ph.D. in the American Natuialiit.

adductor muscle, by snapping together its light valves, thus forcibly ejecting the water within against the water without, flits through, and sometimes even skips upon its native element, liko an aquatic butterfly. But no pedestrian does so in all Mollusca-dom. Why then should not these pedate bivalves, the mussels, walk as others of their own people do? "For want of brains!" says one. You are mistaken, sir. They have brains, the right kind too, and in the right place,—a real pedal nervemass, or ganglion; a little bilobed brain at the very base of the "(understanding" itself, that is, exactly nuder the foot, as was fabled of a very agile dancer, that his brains were in his heels.

Now, if seeing is believing, mussels can walk. We once saw a young brown mussel, of the species Modiola plicatula, about five-eighths of an inch in length, turn his foot to most excellent account. We had pulled the youngster's beard off, and then had deposited him at the bottom of a deep aquarium. The water was probably but poorly aerated, hence he was evidently ill at ease, and to our astonishment he at once began travelling over the pebbly bottom, then up the glass side, with the utmost facility and grace. The foot moved precisely as any univalve gasteropod would do, and with the same easy gliding motion. The movement was continued without interruption until it had reached the surface of the water, a distance of not less than 10in., which, added to the distance travelled over the bottom, was probablyequal to 14in. At the surface it lost no time in spinning its byssus, which it fixed to the side for a permanent abode.

For its lively colours, perhaps ruthlessly, we bad picked this little fellow out of a large family cluster, snugly packed in a hole in one of the piles of the dock. It was a large group of all sizes, literally bound together by the silken cords of—attachment shall we say?

A fellow captive was a full grown, black, edible mussel, torn from its anchorage, a stone near by, at low tide. We afterwards found ensconed in this black shell an amount of intelligence which filled us with astonishment. If his youthful fellow-prisoner could beat him at walking, ho was about to accomplish the feat of climbing to the same position bymeans of a species of engineering of a very high order.

In order the better to understand this singular feat, let us introduce it by the narration of some spider tactics we once witnessed. The insect had captured a large beetle, but could not get it to its web, and seemed indisposed to prey upon it away from its den. It had dragged the prey under the web, which was about two feet above. It ran up to a point close by its web; there it attached a thread by which it speedily descended, and then attached the other end to its booty. Again it ascended, affixed another thread, then descended and affixed to the prey as before. Each thread, in sailor phrase, was made taut. After a good many threads had been in this manner attached, each being stretched tightly, and each pulling a little, the weight was seen to ascend a small fraction of an inch. Again the threads were increased, and again the weight ascended a little more; until at last, after incredible labour, perseverance and skill, the little engineer had the satisfaction of success; for its well-earned booty, with one final, tiny jerk " brought up" at the desired spot. The explanation of all this is simple. Suppose we take a cord of the material known by the ladies under the name elastic, and attach it to an ounce weight. If but very moderately stretched it would certainly pull at least a grain. Supposing it to do that, a second one would pull with equal force, and it would be but a simple estimate to determine how many threads would be required to raise the entire weight. But enough of this. Now for the mussel.

Placed at the bottom of the aquarium, where it had been for a couple of days, it hod succeeded in wriggling itself up to one of the glass sides of the tank. This accomplished it protruded its large foot, stretching it up as high on the glass as it could reach, this organ seemingly adhering very tightly. A little hole opened near the extreme forward end of the foot. This tiny hole was really the extremity of a folded or closed groove. Out of this a drop of white gluten, or mucus, not larger than the head of a pin, was exuded and pressed against the glass. There was then a slight withdrawing of the foot, simultaneously with an unfolding or opening of the groove, which contained, as if moulded there, the already completed delicate thread. This done, the partly contractedfoot (not drawn into its shell at all, be it understood) was again extended, this time a little higher than before. The groove or spinneret was again closed, except the little opening on the surface of the foot, whence another httle drop of mucus appeared, which also was pressed against the glass. Again the foot was withdrawn a little, the lips of the groove unfolded, and the moulded thread set free. This gave thread number two. Each was evidently set at considerable tension. And in this wise, thread after thread was formed and set. I regret that I did not record the exact number, but am sure that it was about twelve or sixteen, and the time occupied was between two and three hours, when lo! up went the mussel, about three-eighths of an inch high. Yes, he was drawn up by his own cords. He was literally lifted from terrajirma. Not fit all suspecting what was to follow I mentally exclaimed "This little fellow knows the ropes."

There was next a period of rest. "Whether it was duo to exhaustion of material, and was meant to allow the secreting gland time to evolve a fresh supply or not, I cannot affirm; but may say that such was my belief, for after an hour or so it set to work again precisely as before, attaching a new cluster of threads. This cluster was set about Sin. higher than the previous one. When this new group of filaments was finished, tho same result followed, another lift of a fraction of an inch, hut not quite so high as the first. I now suspected its motive—the animal was actually in this singular manner attempting to reach the surface. It wanted to take an airing, and was really in a fair way to bring it about.

While setting its third cluster of threads, I foresaw a serious difficulty in the way, and one against which the 6pider never has to contend. It was this: after the third lift had been achieved the threads which had accomplished the first lift had changed direction; that is, the ends of the threads, which had pointed downward when pulling up the mussel, were now pointing upward, and were actually pulling it down. Of course the lowermost thread or threads would exert the most retrograde traction. Thought I, "Sir Mussel-man, you will have to exercise your wits now." I rejoice to say that the ingenious little engineer was complete master of the situation. The difficulty was overcome in this way—as each lowest thread became taut in an adverse direction, it was snapped off at the end attached to the animal. This, as I think, was done by two processes; the one by softening that end of the thread by the animal's own juices, purposely applied, as the pupa in the cocoon moistens its silk envelope, when wishing to soften the fibres, so that it can break a hole through which the imago may emerge; the other by a moderate upward pulling, thus breaking the filament at its weakest point.

The next day our little engineer had accomplished the wonderful feat of climbing to the surface by ropes fabricated during the ascent. Without delay it moored itself securely by a cluster of silken lines at the boundary where sky and water met, and was there allowed to enjoy the airing it had so descr vingly won. Bravo! my little Mussel-man I No acrobat can beat thee on the ropes.

And what are we to say to all this? Blind instinct, forsooth 1 Who believes it? The wise men of the 4ges have written as the tradition of the elders—" byssus-bound," of our Mytilus. But it can moke of its bonds mooring lines of safety against the storm, and with consummate skill can build a silken stairway into its own wished-for elysium of delight. It is some three years since the writer witnessed the facts hero recorded, and to this day the sight of a mussel inspires him with profound reflection on the ways of Him who made these creeping things of the sea.

Note.—It has seemed to the writer, that in the per fection of movement shown by the Modiola plieatula, as given above, a high stage of loot development is indicated, such as would hint at a grade oat-ranking M'/tWuir e AulU. The flgure inserted is that of if. cdulu; but tu process of climbing is the same.—S. L.

THE STORY OF A PIECE OF GRANITE. By J. E. Taylob, F.G.S.

THERE arc few rock substances on the surface of the globe which have received more discussion and been more investigated than myself. I am somewhat proud of the attention I have received in this respect, for most of the leading geologists of every country, for the last century, have devoted themselves to the task of seeking out my antecedents. I am acquainted with a whole library of books, all most learnedly written, and various of them proving the reverse of the other, which have been penned on this inexhaustible subject. Even yet the question can hardly be regarded as finally settled. Every now and then some moot point or another crops up to engage the attention of philosophers, but, thanks to the progress of other sciences, the investigation of these is no longer confined to verbal expressions. It is not a little amusing to remember the hot discussions which were held over me at the beginning of the present century. Philosophers though they professed to be, the disputants resembled political squabblers more than anything else. One set declared I was born amid fire,—the other that I was of purely watery origin. Each believed in their own ip.v dixit, aud, as nothing could bo absolutely proved, hacked their own opinions by personalities. Somehow or other the former sect, who were called Plutonists, got the better of the latter, who were termed Neptunists. (The origin of these phrases my readers will not find it diflicult to understand.) But my Plutonic commentators carried their victory too far. Not content with proving that I was not a mere aqueous rock, they

proceeded to declare I was nothing more nor less than one which had cooled down from a fused condition, something like iron slag; nay, it was even urged that 1 was older than any other rock, and the theorists mapped out an idea—which existed for many years after, chiefly owing to its remarkable novelty—showing how the whole universe wae formerly one great cosmical fog; that this diffused matter was condensed into suns, planets, and satellites, each of which existed for ages in a molten condition, owing to the heat evolved during the process of condensation; that the exterior of each planet cooled during the time which followed, and that granite formed part or whole of this cooled envelope I Such in brief was the orthodox notion of my birth, little more than a quarter of a century ago.

Shall I enlighten my readers a little as to the nature of my mineralogical composition? I feel sure that most of them are acquainted with it already, but, if only for form's sake, I must go through with it again. My name is of Latin derivation, and was given me on account of the granular character presented by my different minerals. Generally speaking, these arc four in number— quartz, felspar, mica, and hornblende. Very frequently there are alto traces of other minerals; hut these are the commonest, and those in fact which make up my hulk. The quartz portion you may tell by its glassy appearance, and usually milkwhite colour; whilst another good test is its snperior hardness. This mineral is almost pnre silica, and is one of the most refrangible of known substances. It can with difficulty be slightly dissolved in hot water, under great pressure ; whilst it requires a great deal of heat to melt it, and, generally speaking, some sort of flux to set it a-going. Tlie uext most abundant mineral in the constitution of myself and relatives (for our name is legion) is that called felspar. Yonr eye may detect it in any mass, by its pink or flesh-colour, whilst it is so soft that you may scratch it with your finger nail. It is owing to the unusual abundance of this mineral that 1 am sometimes so friable or "rotten," as the felspar decomposes, and then causes the other minerals to fall asunder, just as the bricks of a wall would if all the cementing mortar were to decompose away, In many districts, as in Cornwall, where granite comes to the surface and has been subjected to atmospheric wear-uud-tear for hundreds of ages, it is not uncommon to find the line felspar wrtshed into a newer deposit. Such is the well-known " kaolin," or China clay of commerce. The chemical composition of felspar is more complex than that of quartz. For instance, although its commonest elements are silica and alumina,—the former the base of common sand, and the latter of clay,— there are also contained in it more or less of sod:i and potash, lime, magnesia, and iron. Mica, the next commonest mineral I possess, is so well known as hardly to need description. All my readers are surely familiar with the small, thin, silvery-looking scales contained in almost every piece of granite. Its ingredients aro much like those of felspar, only differently mixed. Frequently hornblende is a mineral entering into our composition, and my listeners will readily remember it from its black or dark olive-green colour. When it is very abundant, it produces a rock varying from dark grey to black. A great numlx-r of what may be termed varieties of hornblende are known to mineralogists. Its chemical composition, generally speaking, is about one-half silica, more than a quarter magnesia, and little more than half n quarter lime: besides these, there are usually traces of iron, alumina, and fluoric acid.

I mentioned above that I had many relatives, who were more or less nearly connected (I cannot say by blood, but by mineralogical similarity of composition). These take various names, on account of their leading peculiarities. Among these the commonest is porphyry, which takes its name from the purple variety used by the ancients in making vases, &c. This my hearer may know from the large and distinct crystals, usually of felspar or quartz, which are embedded in the granular matrix. Through porphyry granite passes into oil sorts of allied igneous rocks, such as daystone porphyry, clinkstone porphyry, felspar porphyry, and so on. When hornblende takes the place of mica in the composition of granite, the latter goes by the паше of syenite; when talc supplants mica, the result is called protogine. A fine-grained compound of felspar and granite, with equally minute scales of mica, gives to us the varietal name of pegmatite. According to the number of minerals entering into our composition, I and my relatives are roughly classed as binary, ternary, and quaternary granites. All this detail of structure may sound very dry and tedious ; but it is absolutely necessary to go through with it, if my listeners wish to bo more intimate with me.

Although I have not a distinct recollection of my birth (as indeed, who has? ), yet I have more than a suspicion that such elements as soda, potash, lime, Arc, greatly assisted as fluxes in bringing me into my original molten condition. I have mentioned the great number of relatives which claim near or distant kinship with me, and I have now

only to remark that their affinity to myself has been determined solely by the different cirormstances attending their origin. I distinctly and utterly refute the idea that the first-formed crust of the globe was a granitic one 1 I am fully persuaded it could not possibly have been granite, «rid I will give you my reasons by-andby for this seemingly bold assertion. What that cooled cruet wa«. I doubt if science will ever be able to discover. But the fact that it was not granite does not in the least invalidate the theory that every srm, planet, and satellite was so condensed from nebulous matter. This theory must rest on other gronnds and, singularly enough, additional facts are coming to its support every day. We have not the slightest idea of what the primitive rock or crust of the globe wos. The antiquated notion that it must have been granitic arose out of mistaken associations. It was fonnd that, however old might be а stratified rock, whether containing fossils or not, some variety or another of granite was older still! Hence followed the hasty deduction, that originally one granitic crust encircled the fluid matter of the interior of the earth. It was shown how subsequent rocks were themselves formed out of the wear-and-tear of this granite, how thff latter wo? in many places covered np by its own debris, and how the so-called metamorphic rocks were those first formed as stratified deposits, but altered to their present appearance through the intense heat of the newly-created seas, along whose bottoms they had been elaborated.

All this is wrong, and it behoves me now to descend from the region of pure hypothesis to that of pure fact. It is just possible, speaking generally of all the varieties of my family, that Protogine may be oldest. This, however, has never been thoroughly determined. One of my reasons for believing I could not have required any very great heat to reduce me to the molten condition, and that in this process the agency of water, as well as of heat, was necessary, is as follows:—Many of the larger quartz crystals entering into my composition are hollow. Frequently these hollow's are more or less filled with water. Now, it is a known /act that molten matter at a white heat require its temperature, to be considerably lowered before it can even evaporate the water mechanically mived with it. 7t has been recently shown that crystallized matter which has undergone pure igneous fusion, has usually cavities in its crystals, not containing -water, but either stony matter or a kind o! glass, and, in many cases, even a perfect vacuum. Hence the conclusion is arrived at that in the case of coarsegrained granite, containing much quartz, there is actually more proof of the action of water than of dry, ingneous fusion. It is more than probable, therefore, that pressure, heat, and water combined, in the deeply-seated parts of the earth's crust, would cause the rocks to l>e reduced to a kind of paste, and that this paste would be some variety of granite. I can hardly enter into the nbstruse details of the deductions which have Ы-еп made from the chemical and microscopical examinations of myself and relatives. Suffice it to say tht*y result in proving that prrsitirr, and this, generally speaking, of overlying rocks stratified or otherwise, is a preliminary and i udispensable necessity to the formation of granite; that, if pressure be absent or less than that required, notwithstanding all the other requirements may be present—such as heat, similarity of mineral ingredients, Arc.—such a resulting igneous rock would not be granite! It might be a variety of porphyry, or basalt, or greenstone, or, if all pressure were removed, and the molten matter allowed tti cool in the open air, simply ordinary lava! From a microscopical examination of vorions granites, it has been shown that those of the Highlands of Scotland indicate their having been formed under no less a pressure than 26,000ft. of overlying rocks more than were the granites of Cornwall. There is good reason for believing the latter to have required at least 40,000ft. of roekpressure ; so, in that case, the granites of the Highlands must have been formed when 66,000ft. overlying rocks were piled above them I

One is naturally astounded by the magnitude of these operations, bnt I assure my hearers there is little doubt as to the general correctness of the deductions. In this way tho mineralogical construction of myself and others supplements the teaching of organic remains as to the immense antiquity of the globe! Nothing short of an eternity of time would have sufficed for all the changes which have been rung upon it. There is reason to believe many of my granitic relations are nothing more or less than /r-nw/tW stratified rocks, and their enclosed fossils! As these have been slowly depressed or submerged, so as to bring the lowestseated portions within tho influence of the earth's internal heat, they have been first metamorphosed into a similar condition to gneiss and mica-schist, and, if the sinking went on, have passed through tliis stage into that pasty condition which deprived them of all stratified structure, and converted them into what I am myself! Then succeeded a reversal of the movement; so that this granite would be thrust slowly upwards with all the overlying strata piled above it. The movement went on until these were tilted into a continuous niouutain-cboin.

higli ami extensive table-lands. Meantime the unite nucleus would form the heart of such oixntains, the strata dipping away, as in the inialayas.

I fancy I hear some of my listeners remarking— But if granite can only be formed under such uui'nae pressure, how is it we find such large ■oas of country where nothing else is to be seen? i the answer to this we have the gist of the argu<?t*t, uud I would respectfully ask the special .tentiou of my audience to it. Let them ask them"■l vea where the materials came from to form the inxrentian, Cambrian, Silurian, Devonian, and, in 10rt, all the other subsequent formations? They >uld onlv have been formed out of the waste of ,ill older and already solidified rocks. Each »i-xnation, therefore, represents the amount of -cn.r-aud-tear which went on during the period ,-lien it was deposited. If there had been no oiiipensation against this levelling process, all He prominences would soon have been wom down «> a common level, and the elaboration of more •ecent deposits been self-checked. But each suc•oeding formation shows that this was not the case, ^>ut indicates that the physical arrangements of our planet have been much the same through all tune to what they are at present; that atmospherical and marine wear-and-tear were counterbalanced by upheaval from beneath; that tho exterior force emanating from the sun and resulting ui all these atmospherical effects, was exactly adjusted by the native force of the earth, exerted from the interior outwards. These two have been in equable counterpoiso from the beginning, otherwise the great life-sohetne of our globle would never have had time for its development!

I hope I have been sueoessfnl in explaining a great difficulty, and that my listeners now see the reason why I and my relatives come to the surface. It is because the roeks which overlay mo at my birth liavo since been stripped off and slowly removed by atmospheric agencies. All the formations which were then piled above me, are to be found in stratified rocks of later date; therefore, the period of my birth is not limited to any particular geological epoch. I am found at the surface, surrounded by rocks of every age, even including tho3e of the Tertiary. Wherever the pent-up force of the earth's interior has thrust us up, there have we slowly elevated the rocks lying upon us. In many cases this elevation has been so slow that it has hardly exceeded the rapidity with which these overlying rocks have been denuded away! Think of the vast antiquity of our earth's crust, as indicated by these facts alone! Since the granites of the Highlands of Scotland were formed, twelve miles of overlying material must have been removed! Where has it all gone to? Ask the nineteen miles in vertical thiokness of the known stratified rocks, all of which have probably been formed since the granite itself. We scarcely need be afraid of Time, when we have Eternity to draw upon 1— fiMenee Gossip.

ASTRONOMICAL NOTES FOR SEPTEMBER. By A Fellow of The Royal Astronomical Society. The right ascension of the Buu on September 1st, at Greenwich mean noon is lOh. 41m. 41*22s., and his declination north, 8? 16' 21*3"; he is consequently to the W. of the star \ Leonis (map, Vol. X, p. 545). He is now approaching the equator; uud at 9m. past 6 on the morning of the 28rd, crosses it; when autumn commences. This is, of course, tho period o f one of the equinoxes, so called from the approximate equality in length of the day and night. The nearest approach to such equality will, however, occur on the 25th, when the day will be about 12h. lm. long; and the length of the night obviously llh. 59m. The equation of time to be subtracted from that indicated by any instrument constructed to show the sun's meridian passage, is on the 1st day of the month, 0m. (Ms., and on the last day, 9m. 59-46s.

Tho moon enters her first quarter at 53m. past 1 in the afternoon of tho 2nd; is fall about 12m. past 10 on the night of the 9th; enters her last quarter at h past 1 a.m. on the 18th; and is new at 6h. 34m. a.m. on the 25th. At noon on the 1st she is 5*6 days old; at the same hour on the 2nd, 6*6 days old; and so on. On the afternoon of the (ith, at 8 o'clock, libration will bring an additional portion of her S.W. surface into view, and the samo cause will operate in rendering more of bar S.E. region visible at 1 o'clock in the morning of the 2lBt. She will be in conjunction with Saturn at lOh. 43m. a.m. on the 3rd; with Jupiter at 9m. past 2 in the early morning of the 18th; with Uranus at 53m. after noon on the 20th; with Mars at lOh. 36m. tlie next morning; with Venus at 7h. 13m. in the evening of the 23rd; with Mercury at 8h. 11m. a.m. on the 26th; and finally, with Saturn again at 12m. post 6 in the evening of the 30th. She will have occulted him (as will be mentioned immediately) 8m. previously. Three fixed stars and—as we have just said— Saturn will bo occulted this month. Firstly, B.A.C. 6448 will disappear at the moon's dark limb at 8h. 7m. on the night of the 4th, and reappear at her bright

limb at 9h. 2jjm. On the 13th, P Ceti will disappoar at the moon's bright limb at 9h. 27m. and reappear at her dork limb 6m. afterwards. On the l(tth, ■ Tauri will disappear at the bright limb at lOh. 11m., reappearing at the dark limb at llh. 7m. Lastly, Saturn will disappear at tho dark limb of the moon at eh. 4m. p.m. on the 30th, and will emerge from behind the bright limb at 7h. 18m.

Mercury is an evening Btor daring September, attaining his greatest eastern elongation at 4h. 33m. in the early morning of the 8th. Having passed this he becomes apparently stationary on the 21st. On the 16th, at 4h. 54m. p.m., he will be in conjunction with the bright star a (Spica) Virginia. He continues in the constellation Virgo all this month. He rises not long before b o'clock in the morning at tho beginning of the month, and soon after 7 at the end of it; setting at those periods soon after 7 p.m., and abont 5h. 80m. p.m. respectively. He souths on the 1st at In. 33 n. p.m., and on the 80th at 14m. past noon. Venus is a morning star throughout the month, rising on the 1st about 2h. 50m. a.m., and on the 30th about 4h. 17m. a.m., southing at lOh. 25m. a.m., and 10b. 49m. a.m., and setting about Gh. and 5h. 20tn. p.m. on those days respectively. She travels from Cancer into Leo during September, and is in conjunction with llegulus at th. 48m. a.m. on the 14th. She is in perihelion at 4 in the afternoon of the 20th. Mars is also a morning star, and a little more favourably situated for observation than he has been; albeit he is still au exceedingly minute and insignificant object. He rises somewhere about 8m. past 1 a.m. on the 1st, souths at 8^m. past 9 a.m., and sets about 5h. lGm. p.m. On the 30th he rises about Oh. 48m. a.m., souths at Mh.27m. a.m., and sets about 4h. 9m. p.m. He is in the constellation Cancer during the entire month; and in conjunction with /i2 Caneri at 3 o'clock in the afternoon of the 1st. Jupiter rises on the 1st, about 10b. 31m. p.m., and about 8h. 53m. p.m. on the 30th; after which he is visible all nightlong. He souths on the 1st at 6h. 51m. a.m. and on the last day of the mouth at 5h. 8m. a.m. He still remains in Gemini.

The visible phenomena of his satellites during the month of September will be as follows. On the 1st day of the month, the first satellite will be eclipBed at 12h. 18m. 22s. The shadow of the second will pass on to Jupiter's disc at 48m. past 1 in the early morning of the 2nd; tho first reappeur from behind the planet's body atSh. 49m., and the shadow of the second will pass off Jupiter's limb at 4h. 20m.; 8m. afterward* this second satellite >cill commence its own transit. This, as most of the readers of this column are aware, is referable to the relative position of the Sun, Earth, and Jupiter: in fact, on the 18th of this month, just S7in. before noon, Jupiter will bo in quadrature with the Suu; that is to say, a lino drawn from Jupiter to the Sun will form a right angle with one drawn to the sun from the earth. Any one who will make a diagram of this upon a piece of paper, will see how very obliquely the shadows of Jupiter's satellites will folljupon his disc relatively to onr line of sight, and hence how it is that their shadows may traverse the whole width of that disc beforo the moons which cast them come on to it at all. On the night of the 2nd, at llh. 40m., the egress of the shadow of satellite 1 will take place. The satellite itself will not pass off the disc until 6m. past 1 the next morning. At lh. 51m. in the early morning of the 4th, tho second satellite will reappear from occnltation. On the night of the 5th, at 40m. after midnight, the third satellite will begin to transit Jupiter's disc, and will pass off afterwards at 3h. 12m. On the early morning of the 9th tho first satellite will be oclipsed at 2h. 11m. 45s., and the shadow of the second come on to the planet's limb at 4h. 22m. On tho 9th the ingress of the shadow of satellite 1 will tako place at llh. 26m.; the satellite will come on an hour and twenty minutes later. The shadow will pass off at lh. 40m. a.m. on the 10th, and the moon itself at lm. past 8. On the night of the 10th satellite 2 will be eclipsed at llh. 12m. 23s., and satellite 1 reappear from oconltation at 13m. after midnight. Tho second satellite will reappear from eclipse at lh. 41m. 27s. a.m. on the 11th, only to be occulted by the body of the planet at lh. 54m. It will reappear from this occupation at half past 4. On the 12th the ingress of the shadow of satellite 3 will occur at llh. 25m. and its egress at 48m. past 1 in tho next morning. An eclipse of the first satellite will tako place at 4h. 5m. 8s. a,m. on tho ICth of the month. At lh. 20m. a.m., on the 17th, the ingress of the shadow of the same satellite will take place, tho satellite following its shadow at 2h. 40m. The shadow will pass off at 3h. 84m. Satellite 2 will be eclipsed at lh. 49ra. 3s. a.m. on the 18th, and the first reappear from oconltation at dm. past 2; the second will reappear from its eclipse at 4b. 18m. 29s., and be occulted by the plauet at lh. 32m. a.m. At llh. 24m., on the night of tho same day, the first satellite will pass off Jupiter's disc. On the night of the 19th, the egress of the shadow of satellite 2 will occur at lOh. 40m.; the moon casting it will come on 10m. afterwards, and pass off at lh.33m. the next morning. The ingress of the shadow of satellite 3 will take place at 3h. 25m. Satellite 8 will be occulted at lOh. 42m. on the 23rd, and reappear from occultation at lh, 10m. the next morning. The ingress of the shadow of satellite 1 will take place at 3h. 14m. a.m. on the 24th, and the moon itself follow it at 4h. Sim. At 26m. 52s. past midnight on tho 24th, the first satellite will disappear in eclipse, and afterwards reappear from occultation at 8h. 59m. a.m. on the 25th; the second satellite will be eclipsed at 4h. 25m. S8s. At llh. 2m at night, on the same day, the first satellite will begin to transit Jupiter; its shadow will pass off at 54m. later, the satellite itself leaving the plonet'B limb at 17m. past 1 the next morning. On the

night of tho 2flth satellite 1 will reappear from occultation at lOh. 27m. The Bhodow of satellite 2 will come on to Jupiter's disc at lOh. 46m., and pass off again at lh. 20m. the next morning. Eight minutes afterwards satellite 2 itself will commence its transit and leave the face of the plauet nt 4h. 4m. This some moon will reappear from occultation at 11 o'clock on the night of the 28th. Finally, on the night of the 80th, satellite 3 will reappear from eclipse at llh. 86m. 32s.; will be occulted at 2b. 85m. the next morning, and come out from behind the planet at 5h. 3m.; while 5m. later the shadow of satellite 1 will appear on the disc.

Saturn is still an evening star, but mnst be looked for early, as ho sets between 10 and 11 at tho beginning of tho month, and between 8 and 9 at the end of it. He is on the meridian at Gh. 42m. in the evening of the 1st, and at 4h. 52m. in the afternoon of the 30th. The time has practically past for favourably observing him. Ho remains on the confines of Ophiuchus. Uranns is a morning star, rising about 6m. past 1 a.m. on the 1st, and about llh. 10m. on the last days of the month respectively. He comes on to the meridian in broad daylight all through September, and can only be observed during the hour or two which precede the morning twilight. He is situated throughout September on the confines of Gemini and Cancer. Neptune is not on the meridian until 88m. past 2 on the night of the 1st, but rises about 8 o'clock. He will be a little way to the N. W. of M Piscium during the entire month, his motion being so exceedingly slow.

Shooting stars are rare in September. There is just a suspicion though of the existence of two periodical showerB—one at the beginning of the month, the other in the interval between the 18th and the 25th.

LETTERS TO THE EDITOR.

[We do not hold ourselves responsible for the opinions of our correspondents. The Editoe respectfully requests that all communications should bs drawn up as briefly as possible.]

*,* All communications should be addressed to the Editor of the English Mechanic, 81, TavistockBtreet, Covent Garden, W.C.

All Cheques and Post Office Orders to bo made payable to J. Pas&more Edwards.

"I would have every one write what he knows, and as much as he knowB, but no more; and that not in this only, but in all other subjects: For such a person may have some particular knowledge and oxperience of the nature of such a person or Buch a fountain, that as to other things, knows no more than what evorybody does, and yet to keep a clutter with this little pittance of his, will undertako to write the whole body of physicks: a vice from whence great inconveniences derive their original."—Montaigne's Essays.

*»* Is order to facilitate reference, correspondents when speaking of any Letter previously inserted will oblige by mentioning the number of the Letter, as well as the page on which it appears.

THE HERSCHELIAN TELESCOPE. [289] Sib,—In my reply to Mr. White there is an error. A picture in Ni'chol's " Cyclopaedia of the Physical Sciences," deceived me as to the arrangement actnally adopted by Herschel. In this picture the axis of the eye-piece is not directed as it should be to the cmire of the speculum, but parallel to the axis of the tube. This is tho arrangement corresponding to the theoretically just way of making a Herschelian telescope, the vertex of the paraboloidal surface to which the mirror belongs falling on the edge of the mirror nearest to the eye-piece. Herschel in reality only tilted his mirror, au arrangement obviously imperfect. But ho did not tilt it so that its optical axis passed through the centre of the eye-piece, as in Mr. White's arrangement. Had he done so, he would have had no field of view. He so tilted the mirror that its optical axis had a position mid-way between the axis of the tube and the axis of the oye-pieee (regarded as an indefinitely long straight line). In this way the image formed by the mirror was formed by oblique pencils.

Richard A. Proctor.

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Firth to continue his absurd comments on the valuable Astronomical Notes by "F.R.A.S." All the readers of the English Mechanic owe a debt of gratitude to "F.R.A.S." for the preparation of these most useful notes. But we cannot expect him to contribnte so much of his time for our advantage if his labours are to be continually carped at by some who seem unable to appreciate their value.

Mr. Firth's last letter reminds me of an episode in "David Copperfield." Mr. Dick has been installed into apartments somewhat limited in extent, bat sufficient for hiB requirements ; Mrs. Crupp, however, remarks that "there's not room to swing a cat," to which Mr Dick makes the apt rejoinder, that "he doesn't want to swing a cat, so what can that matter to him?" We have been supplied by "F.R.A.S." with information trne for the present time, but not for all time. Mr. Firth, however, remarks that thousands of years hence the state of the case will be different; may not we readers of the English Mechanic rejoin (not inaptly) that " we don't want to know what will happen thousands of years hence, so what can that matter to us?"

The fact is that when the Nautical Almanac says that a celestial object at a given date and hour will have a certain longitude and latitude, it in effect asserts, as "F.R.A.S." has said, that that object will be in a certain part of such and such a constellation, the date and the hour are as truly parts of the statement as the longitude and the latitude. If it were not so, Mr. Firth would not be j ustified in rudely telling "F.R.A.S." that he is talking nonsense, because it would at least be obvious that "F.R.A.S." had been trying to render us a service, and therefore that any error he might unwittingly hare made could not be too courteously and kindly corrected. But as a matter of fact " FJt.A.8." was wholly in the right.

Mr. Firth should endeavonr to understand that men of "F.R.A.S.'s" calibre do not write in our columns to raise discussion, nor because of any pleasure it gives them to see their views in print (those who so write cannot be too severely handled): "F.R.A.S." writes, we all feel, from a disinterested desire to impart useful information. Nothing is more likely to discourage him in his exertions than to see his communications made a reason for twaddling comments by those who cannet appreciate the spirit in which he writes.

"F.R.A.S." has suggested that Mr. Firth had better 'to the astronomical notes himself. We are all interested in loudly protesting against any such arrangement. Bat I have the proposal to make which will no doubt suit all parties. Let Mr. Firth devote his energies to describing the state of the heavens 10,000 years hence; and let him give directions that his papers shall be reserved until the epoch they refer to.

Richard A. Proctor.

THE NATURE AND CAUSE OF LIGHT.

[291] Sir,—I am sorry to Bee that " T. A." has not taken in good part my suggestion that his views about light are unsound. I did not make that suggestion without a purpose. Those who "confuse counsel by words without knowledge" must not complain if they meet with a check. "T. A." should remember how large a proportion of our readers trust only to those who know to save them from being misled and confused by those who do not know. In saying this I am not claiming anything on my own account; I have not found out for myself what I know about light, but have learned it from the careful study of the work (the observations, experiments, writings, and so on) of other men; and I knew that most of "T. A.'s" views were opposed altogether to the whole series of results obtained by the scientific students of the subject he treated of. I held it to be important that the readers of our journal should also know this; and therefore, without any ill-feeling at all towards "T. A.," I said as much.

Let me add that he had been very blunt himself in contradicting me about the "crepuscular curve," whose existence he abruptly denied. It turned out that he was simply unfamiliar/with the meaning of that well-known expression. But I may take this opportunity of admitting the justice of his subsequent criticism. Since the words "twilight curve " express exactly the same as " crepuscular curve," and are much less likely to be misunderstood, I ought to have used them. I make a rule of using the simplest words I can find; but in this case, by inadvertence- used the less familiar expression.

Now as to the first question " T. A." asks, I must remind him that it belongs rather to the subject of chemistry than of light considered in the abstract. But apart from this, we do not yet know why light from the violet end of the spectrum should have the power of producing chemical action (of causing the combination or decomposition of certain substances), while light from the red end has no such power. How, or why, then, should I be expected to explain a well-known but still more recondite phenomenon ?" T. A." might as reasonably say, "Mr. Proctor professes to have some knowledge of the laws of light and colour, let him tell me, then, why some people have red hair, others black, and yet others brown or flaxen.'' "T. A." some time back called on me and on our " F.R.A.S." par excellence, to answer two questions of similar profundity. He was invited to reflect on an old proverb about questions and answers as related to wisdom and folly. Truly, without reflecting on "T. A.'s " wisdom in asking this particular question, I must admit that I should lie but foolish did I pretend to answer it. That it will be answered same day I make no question, but years of patient labour must I believe come first. At present let us content ourselves with stating what the experiment shows. First of all, light, passing through an orange-yellow glass will not affect a sensitized collodion plate. That was to be expected, because little or no light from the violet end of the spectrum can pass through such glass (as we know from the absorption spectra of orangeyellow glasses). Light waves of more than a certain length cannot produce the required chemical action, then, it appears. But after the collodion film has been subjected-to the action of actinic light, orange-yellow light has the power of destroying the impression and fogging the plate all over. A most interesting fact, serving to show that after the arrangement of the molecules of the film has been affected in a certain way, by the shorter actinic waves, the longer light waves have the power of further influencing the arrangement of

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those molecules. So much for the how, the why is quite another matter. The second question I leave untouched, not knowing the exact arrangements adopted while the successive pictures are taken.

"T. A.'s " remark about the zodiacal light is perplexing. I thought I had dwelt in a manner there was no mistaking on the very theory he says I have omitted to refer to. Richard A. Proctor.

P.S.—There is an erratum, probably due to a slip of tho pen, in my letter on "the Satellites of Uranus." The words "position angle of Uranus" should be "position angle of a satellite."

MONCRIEFF'S HYDRO-PNEUMATIC GUN-
CARRIAGE.

[292] Sir,—I send you a drawing, taken from Engineering, showing Captain Moncricfl's improved gun carriage in all its details, and indicating both the loading and firing position of the gun, which is mounted upon a revolving carriage. The circular travel described by the wheels upon the lower deck is 12ft. trim, and at the upper deck the framework is free to turn round an inclined path 17ft. 6in. diameter, upon which rollers set at an angle take their bearing, the revolving motion being effected by bevel gearing, as shown. Under the carriage is placed an hydraulic cylinder, the ram of which has a T'shaped head, and is provided with small rollers which bear upon the under side of the moving part of the carriage. In the lower part of the carriage, that which has no movement except a circular one, a vertical opening is left one each side, as shown, and these serve as guides for the ascending or descending ram, the end of the Thead, projecting through the openings on either side. Parallel links, the position of which, when the gun is in firing position, is vertical, are secured at the lower end to the bottom of the fixed part of the carriage, and at the upper end to the movable part, their motion being the same as the links in a parallel ruler, as the gun rises or falls. Connected with tho hydraulic cylinder is a,pipe leading to an air vessel, and having a valve chamber containing a spherical valve. A bye-pass pipe, which can be opened or closed by a lever from the gun platform, establishes an independent communication between the air chamber and that portion of the main pipe between the valve chamber and the hydraulic cylinder. In the rear of the air chamber is a small pipe for supplying water deficiencies by leakage. The action of the mechanism is as follows :—Water is pumped into the apparatus until the air in the air chamber is placed under a considerable pressure. When the gun is loaded, and it is desired to raise it, the opening of the bye-pass establishes a communication with the hydraulic cylinder, the ram of which rises carrying with it the gun. The valve is then closed, and when the piece is fired the recoil throws it back with a constantly decreasing velocity, due partly to the increasing resistance of the coupling links and partly to the increasing pressure within the air chamber Kappa.

MOUNTING INSECTS FOR THE HKWHMfc

[298] Sir,—Allow me to recommeftA *(ktam* catch every insect that comes in his way, to eau&u* same to his death-bottle, and to ftftcrWril a**! The fact is, " a list of objects easily obtained " *^ occupy two or three numbers of our Micsis^ i space which I think would hardly lie aliens i. no. so discerning a body as the microscopists.

Perhaps " Q. Sea" would say that such a proes*^ would doubtless give him a large supply of sabjw* 'mounting, but how about the names of the ts--' caught? These can easily be learnt by means ^> book on entomology; and, indeed, not only the wc but also the various organs, Ac., by which the cl which the insects under examination belong hi tinguished; and thus the operator knows whette look for a gizzard, or a padded, or n. hooked foot: or not. And, last but not least, a considerable iz of scientific knowledge of entomology is gained.

Concerning the mode of mounting (I say "V because I have as yet spoken only of insects, aiw ~of these as should be mounted by the mode undff* cussion would be quite enough to fully occupy tb§a of B beginner for some time to come; and there*? prefer classifying objects according to their nwfa>: preparation, not necessarily of mounting andtr*^ of them separately), the following I find very t±r both for very large whole insects, and for nncnt C' sects, or only parts of them.

The insect having been caught, is introdnctd e*; "death-bottle " (the modus operandi of vSaioh wisp'** in a previous'number), and when dead is di**according to what parts are wished to be pre*"7-and then put into a rather weak solution of c*» potash, made by adding to the liquor potass '' chemist about its own bulk of water. In this so'? the future object must be allowed to remain t fortnight to three months, according to the Uxtv* sire of subject. When the object has been consi»to have remained long enough in the caustic (ac«: this point a little consideration will be quite suftft": guide, and, indeed, it does not matter if an Obie', left a matter of two or three days beyond the necessary time, more especially if to this solution of poti about one half its bulk of methylated spirit be add* it should be taken out and thrown into water (a* P as possible) and allowed to remain there for four-w twenty hours, the water being changed at least m during this period. Takea out of the water, the obi must be brushed clean with a camel's-hair brush a a plentiful supply of clean water; it is then arrange on a glass slide according to taste; another slide is | on the top, and the whole squeezed firmly between strong-springed American clothes-peg, or a letter) and in this position is allowed to dry for a few day? a week, according to texture, time, Ac., of object. Wn dry, the object is carefully removed from the slide, a immersed in strong methylated spirit, to extract t last trace of moisture, and there left for a peri varying according to circumstances. From the Bpi the object should go into ether (methylated will <3 and should be left there for three or four days. At t

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