deductions from them; and whenever a violent adductor muscle, by snapping together its light storm was in progress, to decide in what direction valves, thus forcibly ejecting the water within and with what velocity it was travelling; determine against the water without, flits through, and somewhat places it would visit, and at what hour it would times even skips upon its native element, like an arrive and finally transmit the announcement aquatic butterfly. But no pedestrian does so in all immediately by telegraph to those places especially Mollusca-dom. Why then should not these pedate interested. Such a system could not be expected bivalves, the mussels, walk as others of their own to attain satisfactory results without a pretty large people do ? "For want of brains!" says one. You number of well-selected stations, and especially are mistaken, sir. They have brains, the right without the service of a competent meteorologist kind too, and in the right place, a real pedal to superintend the entire system. The superin- nervemass, or ganglion; a little bilobed brain tendent should be well informed respecting the at the very base of the "understanding" itself, progress which has been already made in this that is, exactly nuder the foot, as was fabled of department of science; he should have strong a very agile dancer, that his brains were in his faith in the practicability of attaining useful results heels. by a system of storm-warnings; 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."

about three-eighths of an inch high. Yes, he was
drawn up by his own cords. He was literally lifted
from terra firma. Not at 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
due 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 ĝin.
higher than the previous one. When this new
group of filaments was finished, the same result
followed, another lift of a fraction of an inch, but
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 spider 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.

proceeded to declare I was nothing more nor less only to remark that their affinity smal than one which had cooled down from a fused con- been determined solely by the ener dition, something like iron slag; nay, it was even stances attending their origin. Idothe urged that I was older than any other rock, and the terly refute the idea that the first-lottheorists mapped out an idea-which existed for the globe was a granitic one! many years after, chiefly owing to its remarkable suaded it could not possibly have her novelty-showing how the whole universe was for. I will give you my reasons by-and-by art, merly one great cosmical fog; that this diffused ingly bold assertion. What that cus matter was condensed into suns, planets, and satel- I doubt if science will ever be able wi lites, each of which existed for ages in a molten But the fact that it was not granite does y condition, owing to the heat evolved during the least invalidate the theory that every e process of condensation; that the exterior of each and satellite was so condensed fr planet cooled during the time which followed, and matter. This theory must rest on ette that granite formed part or whole of this cooled and, singularly enough, additional fact envelope! Such in brief was the orthodox notion to its support every day. We have not of my birth, little more than a quarter of a century idea of what the primitive rock or ago. globe was. The antiquated notion t Shall I enlighten my readers a little as to the have been granitic arose out of mist nature of my mineralogical composition? I feel tions. It was found that, however el sure that most of them are acquainted with it stratified rock, whether containing fort already, but, if only for form's sake, I must go some variety or another of granite we through with it again. My name is of Latin deri. Hence followed the hasty deduction, th vation, and was given me on account of the granu- one granitic crust encircled the fluid lar character presented by my different minerals. interior of the earth. It was shows Generally speaking, these are four in number-quent rocks were themselves formed « quartz, felspar, mica, and hornblende. Very fre- wear-and-tear of this granite, how the g quently there are also traces of other minerals; in many places covered up by its own deze but these are the commonest, and those in fact the so-called metamorphic rocks were the which make up my bulk. The quartz portion you formed as stratified deposits, but altere may tell by its glassy appearance, and usually milk-present appearance through the intense he white colour; whilst another good test is its newly-created seas, along whose bottoms te superior hardness. This mineral is almost pure been elaborated. silica, and is one of the most refrangible of known All this is wrong, and it behoves me substances. It can with difficulty be slightly descend from the region of pure hypothesis t dissolved in hot water, under great pressure; whilst of pure fact. It is just possible, speaking rese it requires a great deal of heat to melt it, and, of all the varieties of my family, that Pre generally speaking, some sort of flux to set it may be oldest. This, however, has aver a-going. The next most abundant mineral in the thoroughly determined. One of my reasons i constitution of myself and relatives (for our name believing I could not have required any very goo is legion) is that called felspar. Your eye may heat to reduce me to the molten con-linen, and the detect it in any mass, by its pink or flesh-colour, in this process the agency of water, as well as whilst it is so soft that you may scratch it heat, was necessary, is as follows-Many of the with your finger nail. It is owing to the unusual larger quartz crystals entering into a composition abundance of this mineral that I am sometimes are hollow. Frequently these holon's are pere er so friable or "rotten," as the felspar decom- less filled with water. Now, it known fact that The next day our little engineer had accomplished poses, and then causes the other minerals to fall molten matter at a white beat recomms its tempera the wonderful feat of climbing to the surface by asunder, just as the bricks of a wall would if all ture to be considerably lewens i bet re it can even ropes fabricated during the ascent. Without delay the cementing mortar were to decompose away, evaporate the water mere moved with it. It it moored itself securely by a cluster of silken lines In many districts, as in Cornwall, where granite has been recently show that crystalized matter at the boundary where sky and water met, and was comes to the surface and has been subjected to at- which has undergone pare into tison has there allowed to enjoy the airing it had so deser-mospheric wear-and-tear for hundreds of ages, it is usually cavities in its crystals, na e' inne wild, vingly won. Bravo! my little Mussel-man! No not uncommon to find the fine felspar washed into but either stony matter or a kid gas, and, in acrobat can beat thee on the ropes. a newer deposit. Such is the well-known" kaolin," many cases, even a perfect van Heuer the And what are we to say to all this? Blind in- or China clay of commerce. The chemical com- conclusion is arrived at that in the case of course. stinct, forsooth! Who believes it? The wise men position of felspar is more complex than that of grained granite, containing much quartz, there is of the ages have written as the tradition of the quartz. For instance, although its commonest actually more proof of the action of water than d elders-" byssus-bound," of our Mytilus. But it elements are silica and alumina,-the former the dry, ingneous fusion. It is more than provable can make of its bonds mooring lines of safety base of common sand, and the latter of clay, therefore, that pressure, heat, and water combined against the storm, and with consummate skill can there are also contained in it more or less of soda in the deeply-seated parts of the earth's res build a silken stairway into its own wished-for and potash, lime, magnesia, and iron. Mica, the would cause the rocks to be reduced to a kind elysium of delight. It is some three years since next commonest mineral I possess, is so well paste, and that this paste would be some variety the writer witnessed the facts here recorded, and to known as hardly to need description. All my granite. I can hardly enter into the abstruse de this day the sight of a mussel inspires him with readers are surely familiar with the small, thin, of the deductions which have been made from profound reflection on the ways of Him who made silvery-looking scales contained in almost every chemical and microscopical examinations of mys these creeping things of the sea. piece of granite. Its ingredients are much like and relatives. Suffice it to say they result in those of felspar, only differently mixed. quently hornblende is a mineral entering into our overlying rocks stratified or otherwise, is a pr Fre- ing that pressure, and this, generally speak composition, and my listeners will readily remem- liminary and indispensable necessity to the form ber it from its black or dark olive-green colour. of granite; that, if pressure be absent or less When it is very abundant, it produces a rock vary that required, notwithstanding all the other ing from dark grey to black. A great number of what quirements may be present-such as heat, s may be termed varieties of hornblende are known of mineral ingredients, &c.-such a resulting to mineralogists. Its chemical composition, gene- rock would not be granite! It might be a rally speaking, is about one-half silica, more than a of porphyry, or basalt, or greenstone, fi quarter magnesia, and little more than half a pressure were removed, and the moltes quarter lime: besides these, there are usually allowed to cool in the open air, simply traces of iron, alumina, and fluoric acid. lava! From a microscopical examinati I mentioned above that I had many relatives, various granites, it has been shown that the who were more or less nearly connected (I cannot the Highlands of Scotland indicate their say by blood, but by mineralogical similarity of been formed under no less a pressure than ** composition). These take various names, on of overlying rocks more than were the grue account of their leading peculiarities. Among Cornwall. There is good reason for belie these the commonest is porphyry, which takes its latter to have required at least 40,000. name from the purple variety used by the ancients pressure; so, in that case, the granites of th in making vases, &c. This my hearer may know lands must have been formed when 669 from the large and distinct crystals, usually of overlying rocks were piled above them! felspar or quartz, which are embedded in the granular matrix. Through porphyry granite passes into all sorts of allied igneous rocks, such as claystone 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 name of syenite; when tale 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 varictal 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 be 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, &c., greatly assisted as fluxes in bringing me Plutonic commentators into my original molten condition. I have mencarried their victory too far. Not content with tioned the great number of relatives which claim proving that I was not a mere aqueous rock, they near or distant kinship with me, and I have now

NOTE. It has seemed to the writer, that in the perfection of movement shown by the Modiola plicatula, as given above, a high stage of foot development is indicated, such as would hint at a grade out-ranking Mytilus e dulis. The figure inserted is that of M. edulis; but the

process of climbing is the same.-S. L.

THE STORY OF A PIECE OF GRANITE.
BY J. E. TAYLOR, F.G.S.

HERE are 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 ipse dixit, and, as nothing could be absolutely proved, backed 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 difficult to understand.) But my

#5

these operations, but I assure my hearer
One is naturally astounded by the ma
little doubt as to the general correcte
deductions. In this way the minerals
struction of myself and others supplem
teaching of organic remains as to the i
antiquity of the globe! Nothing short of ane
of time would have sufficed for all the cha
which have been rung upon it.
believe many of my granitic relations are noth
There is reas
more or less than re-melted stratified rocks. ►
their enclosed fossils! As these have been sh
depressed or submerged, so as to bring the low
seated portions within the influence of the eart
internal heat, they have been first metamorphos
into a similar condition to gneiss and mica-schie
and, if the sinking went on, have passed thron.
this stage into that pasty condition which deprive
them of all stratified structure, and converted the
into what I am myself! Then succeeded a revers
of the movement: so that this grauite would b
thrust slowly upwards with all the overlying strat
piled above it. The movement went
these were tilted into a continuous mountain-chair

r high and extensive table-lands. Meantime the ranite nucleus would form the heart of such ountains, the strata dipping away, as in the Himalayas.

limb at 9h. 25m. On the 13th, 2 Ceti will disappear
at the moon's bright limb at 9h. 27m. and re-
appear at her dark limb 6m. afterwards. On the
16th, Tauri will disappear at the bright limb at
10h. 11m., reappearing at the dark limb at 11h. 7m.
Lastly, Saturn will disappear at the dark limb of the
moon at 6h. 4m. p.m. on the 30th, and will emerge
from behind the bright limb at 7h. 18m.

I fancy I hear some of my listeners remarking But if granite can only be formed under such mmense pressure, how is it we find such large reas of country where nothing else is to be seen?" Mercury is an evening star during September, atn the answer to this we have the gist of the argu-taining his greatest eastern elongation at 4h. 33m. in nent, and I would respectfully ask the special the early morning of the 8th. Having passed this he ttention of my audience to it. Let them ask them- becomes apparently stationary on the 21st. On the selves where the materials came from to form the 16th, at 4h. 54m. p.m., he will be in conjunction with Laurentian, Cambrian, Silurian, Devonian, and, in the bright star a (Spica) Virginis. He continues in short, all the other subsequent formations? They the constellation Virgo all this month. He rises not could only have been formed out of the waste of long before 8 o'clock in the morning at the beginning still older and already solidified rocks. Each of the month, and soon after 7 at the end of it; setting formation, therefore, represents the amount of at those periods soon after 7 p.m., and about wear-and-tear which went on during the period 5h. 30m. p.m. respectively. He souths on the 1st at 1h. 33.n. p.m., and on the 30th at 14m. past noon. when it was deposited. If there had been no Venus is a morning star throughout the month, rising compensation against this levelling process, all on the 1st about 2h. 50m. a.m., and on the 30th about the prominences would soon have been worn down 4h. 17m. a.m., southing at 10h. 25m. a.m., and to common level, and the claboration of more 10h. 49m. a.m., and setting about 6h. and 5h. 20m. p.m. recent deposits been self-checked. But each sucon those days respectively. She travels from Cancer ceeding formation shows that this was not the case, into Leo during September, and is in conjunction with but indicates that the physical arrangements of our Regulus at 4h. 48m. a.m. on the 14th. She is in planet have been much the same through all time perihelion at 4 in the afternoon of the 20th. Mars is to what they are at present; that atmospherical also a morning star, and a little more favourably and marine wear-and-tear were counterbalanced by situated for observation than he has been; albeit he is upheaval from beneath; that the exterior force still an exceedingly minute and insignificant object. emanating from the sun and resulting in all these He rises somewhere about 3m. past 1 a.m. on the 1st, atmospherical effects, was exactly adjusted by the souths at 81m. past 9 a.m., and sets about 5h. 16m. native force of the earth, exerted from the interior p.m. On the 30th he rises about 0h. 48m. a.m., souths outwards. These two have been in equable counter- at 8h. 27m. a.m., and sets about 4h. 9m. p.m. He is in poise from the beginning, otherwise the great the constellation Cancer during the entire month; and life-scheme of our globle would never have had time afternoon of the 1st. Jupiter rises on the 1st, about in conjunction with 2 Caneri at 3 o'clock in the J for its development! 10h. 31m. p.m., and about 8h. 53m. p.m. on the 30th; after which he is visible all night long. He souths on the 1st at 6h. 51m. a.m. and on the last day of the month at 51. 8m. a.m. He still remains in Gemini.

a

I hope I have been successful 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 rocks which overlay me at my The visible phenomena of his satellites during the birth have since been stripped off and slowly re- month of September will be as follows. On the 1st moved by atmospheric agencies. All the formations day of the month, the first satellite will be eclipsed at which were then piled above me, are to be found in 12h. 18m. 22s. The shadow of the second will pass on to stratified rocks of later date; therefore, the period Jupiter's disc at 48m. past 1 in the early morning of of my birth is not limited to any particular geological the 2nd; the first reappear from behind the planet's epoch. I am found at the surface, surrounded by body at 3h. 49m., and the shadow of the second will pass rocks of every age, even including those of the off Jupiter's limb at 4h. 20m.; Sm. afterwards this This, as Tertiary. Wherever the pent-up force of the earth's second satellite will commence its own transit. most of the readers of this column are aware, is interior has thrust us up, there have we slowly ele-referable to the relative position of the Sun, Earth, vated the rocks lying upon us. In many cases this elevation has been so slow that it has hardly ex-37m. before noon, Jupiter will be in quadrature with and Jupiter: in fact, on the 18th of this month, just ceeded the rapidity with which these overlying rocks the Sun; that is to say, a line drawn from Jupiter to have been denuded away! Think of the vast the Sun will form a right angle with one drawn to the antiquity of our earth's crust, as indicated by these sun from the earth. Any one who will make a diagram facts alone! Since the granites of the Highlands of this upon a piece of paper, will see how very of Scotland were formed, twelve miles of over- obliquely the shadows of Jupiter's satellites will fall upon lying material must have been removed! Where his disc relatively to our line of sight, and hence how has it all gone to? Ask the nineteen miles in it is that their shadows may traverse the whole width vertical thickness of the known stratified rocks, of that disc before the moons which cast them come on all of which have probably been formed since to it at all. On the night of the 2nd, at 11h. 46m., the the granite itself. We scarcely need be afraid egress of the shadow of satellite 1 will take place. The of Time, when we have Eternity to draw upon!-satellite itself will not pass off the disc until 5m. past Science Gossip. 1 the next morning. At 1h. 51m. in the early morning of the 4th, the second satellite will reappear from occultation. On the night of the 5th, at 46m. after midnight, the third satellite will begin to transit Jupiter's disc, and will pass off afterwards at 3h. 12m. On the early 2h. 11m. 45s., and the shadow of the second come on to the planet's limb at 4h. 22m. On the 9th the in11h. 26m.; the satellite will come on an hour and gress of the shadow of satellite 1 will take place at twenty minutes later. The shadow will pass off at 1h. 40m. a.m. on the 10th, and the moon itself at 1m. past 3. On the night of the 10th satellite 2 will be eclipsed at 11h. 12m. 235., and satellite 1 reappear from occultation at 13m. after midnight. The second satellite will reappear from eclipse at 1h. 41m. 27s. a.m. on the 11th, only to be occulted by the body of the planet at 1h. 54m. It will reappear from this occultation at half past 4. On the 12th the ingress and its egress at 48m. past 1 in the next morning. An of the shadow of satellite 3 will occur at 11h. 25m., eclipse of the first satellite will take place at 4h. 5m. 8s. a.m. on the 16th of the month. At 1h. 20m. a.m., on the 17th, the ingress of the shadow of its shadow at 2h. 40m. the same satellite will take place, the satellite following The shadow will pass off at 3h. 34m. Satellite 2 will be eclipsed at 1h. 49m. 3s. a.m. on the 18th, and the first reappear from occultation at 6m. past 2; the second will reappear from its eclipse at 4h. 18m. 29s., and be occulted by the planet at 4h. 32m. a.m. At 11h. 24m., on the night of the 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 10h. 46m.; the

ASTRONOMICAL NOTES FOR SEPTEMBER. BY A FELLOW OF THE ROYAL ASTRONOMICAL SOCIETY. THE right ascension of the sun on September 1st, at Greenwich mean noon is 10h. 41m. 41-22s., and his declination north, 8 16' 21-3"; he is consequently to the W. of the star x Leonis (map, Vol. X, p. 545). He is now approaching the equator; and at 9m. past 6 on the morning of the 23rd, crosses it; when autumn com

mences.

This is, of course, the period of 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. 1m. long; and the length of the night obviously 11h. 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, Om. 6'1s., and on the last day, 9m. 59-46s.

The moon enters her first quarter at 58m. past 1 in the afternoon of the 2nd; is full about 12m. past 10 on the night of the 9th; enters her last quarter at past 1 a.m. on the 18th; and is new at Ch. 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 6th, at 3 o'clock, libration will bring an additional portion of her S.W. surface into view, and the same cause will operate in rendering more of her S.E. region visible at 1 o'clock in the morning of the 21st. She will be in conjunction with Saturn at 10h. 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 10h. 36m. the next morning; with Venus at 7h. 13m. in the evening of the 23rd; with Mercury at 3h. 11m. a.m. on the 26th; and finally, with Saturn again at 12m. past 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 saidSaturn will be 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

morning of the 9th the first satellite will be eclipsed at

moon casting it will come on 10m. afterwards, and pass off at 1h. 33m. the next morning. The ingress of the shadow of satellite 3 will take place at 3h. 25m. Satellite 8 will be occulted at 10h. 42m. on the 23rd, and reappear from occultation at 1h. 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. 34m. At 26m. 52s. past midnight on the 24th, the first satellite will disappear in eclipse, and afterwards reappear from occultation at 3h. 59m. a.m. on the 25th; the second satellite will be eclipsed at 4h. 25m. 38s. At 11h. 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 planet's limb at 17m. past 1 the next morning. On the

night of the 26th satellite 1 will reappear from occultation at 10h. 27m. The shadow of satellite 2 will come on to Jupiter's disc at 10h. 46m., and pass off again at 1h. 20m. the next morning. Eight minutes afterwards satellite 2 itself will commence its transit and leave the face of the planet at 4h. 4mm. This same moon will reappear from occultation at 11 o'clock on the night of the 28th. Finally, on the night of the 30th, satellite 3 will reappear from eclipse at 11h. 36m. 32s.; will be occulted at 2h. 35m. the next morning, and come out from behind the planet at 5h. 8m.; while 5m. later the shadow of satellite 1 will appear on the disc.

Saturn is still an evening star, but must be looked for early, as he sets between 10 and 11 at the beginning of the month, and between 8 and 9 at the end of it. He is on the meridian at 6h. 42m. in the evening of the 1st, and at 4h. 52m. in the afternoon of the 30th. The time has practically past for favourHe remains on the confines of ably observing him. Ophiuchus. Uranus is a morning star, rising about 6m. past 1 a.m. on the 1st, and about 11h. 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 38m. 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 a Piscium during the entire month, his motion being so exceedingly slow.

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

LETTERS TO THE EDITOR.

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of our correspondents.
The EDITOR respectfully
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EDITOR of the ENGLISH MECHANIC, 31, Tavistockstreet, Covent Garden, W.C.

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

In 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] SIR,-In my reply to Mr. White there is an error. A picture in Nichol's "Cyclopædia of the Physical Sciences," deceived me as to the arrangement actually adopted by Herschel. In this picture the axis of the eye-piece is not directed as it should be to the centre of the speculum, but parallel to the axis of the tube. This is the arrangement corresponding to the theoretically just way of making a Herschelian telescope, the vertex of the paraboloidal surface to which the to the eye-piece. mirror belongs falling on the edge of the mirror nearest Herschel in reality only tilted his mirror, an arrangement obviously imperfect. But he 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 eye-piece (regarded as an indefinitely long straight line). In this way the image formed by the mirror was formed by oblique pencils. RICHARD A. PROCTOR.

MR. FIRTH AND "F.R.A.S." [290] SIR,-I really think that it is too bad of Mr. 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 contribute 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, but sufficient for his 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