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evident that the force of the current is proportional to the sino of the angle the needle then makes with the magnetic meridian. The action of galvanometers based on this principle is more uniform than that of the tangent instrument. because while the sine of one degree is the same as the tangent, that of 90 degrees is 1, the same as the tangent of 45 degrees, and the sines gradually diminish in proportionate length, instead of, as in the case of the tangents, increasing so rapidly as soon to become almost useless as measures of action. Bnt, on the other hand, their action is very limited, as the force should not be able to deflect beyond 90 degrees. A sine galvanometer, therefore, to be of any extended use, should be made with connections enabling each successive turn or layer of wire to be brought into circuit.
Fig. 41 shows the construction in its simplest
form. A is the stand mounted on levelling screws n; to its bottom is fixed a brass spring b, with a clamp screw; its middle corresponds with the central line, and its edge is graduated as described, section 156, in the tangent galvanometer, so as to divide the degrees; in the centre is n strong brass point on which turns freely and smoothly the circle B, which is the base of the instrument itself; its edge is graduated to degrees, and с is a handle by which to move it. On it is fixed C, a wooden frame on which is wound the wiro; n « is the needle, which in this instrument should be heavy and highly magnetized and 4in. to 6in. long, fitted with agate centre to diminish friction, and having fixed to it at right angles a light aluminum or other indicator with a line marked on its middle; e is a point fixed to the stand on the line of DO degrees; + and — arc the binding screws. As before stated, to make a really useful instrument, there will be required several of these for different lengths of wire, or a commutator to serve the same purpose; and it would be more convenient also to have the binding screws on A, so as to remain fixed, and communicating by means of springe pressing on metallic bands on B, which would convey the current to the wire; but theee are modifications readily devised, which cannot be shown on a figure, or without complicated diagrams unsuited to the scope of these papers. The indicator d, may be so short as to pass into the coil or stops provided to arrest the needle, and a glass cover should be provided to keep the needle from being affected by wind and otherwise protect it.
To use the instrument it is placed in the magnetic meridian with e pointing truly to the line on if, and the zero or 0 on the circle at the middle lino of b, or if not, then the deviation is to be noted and applied to the final result. The connection is now made, and В turned round till d iipain rests opposite e; the angle through which В has been moved gives by its sine the proportionate value of the current, and by constructing a table and malting one or two trials, as explained for the tangent instrument, the actual values may be obtained once for all. Of course if the current is powerful it may carry the needle round the whole circle, but it is evident that if it goes over 90 degrees the observation is useless, nrul the instrument too powerful, and hence the necessity for the power to use only one or more turns of the wire, for each of whioh lengths a meoial column will be needed in the table for the actual value based on experiment with that length.
Iivi,—Ordinary galvanometers are of more simple construction, but give no definite informa
tion. It is. however, quite easy to obtain the real value of their deflections, by once carefully comparing them with a sine or tangent instrument. The process is to connect one of the screws with the positive pole of a constant battery, such as a Daniel or Grove, the other being connected to one screw of the other galvanometer and through it to the negative pole of tho battery. The instruments should be so far apart that their needles do not mutually affect each other. A table of degrees and their values being prepared, the degree marked by the instru;in-nt to be tested is entered opposite to that shown by the standard instrument; by increasing the length of the connecting wires and using fine German-silver wire, these deflections may be reduced gradually, and by thus bringing the needle to zero and noting each stage a table is constructed, which will always give the value of the deflections of the instrument, providing the same needle be always used with it. For convenience it is best to note upon the face of each galvanometer the side to which я current deflects the needle, or, if several instruments are possessed, it is still better so to arrange the connections that all deflect the same way—for instance, if the screws are on the sides of the zero line, make the north pole always turn to the side connected to the positive pole of the battery. Columns V., VI., and VII. in the table, p. 530, are the relative deflections taken to the nearest degree of three of my galvanometers, as an example of this mode of valuing them.
161.—Fig. 45 shows the simplest possible form, consisting of a mere rectangle of stout wire encircling the needle. The lower branch should
be let into the wood frame, or even be on the lower side, so as to allow the graduated arc to be on tho wood, and the needle should either curve down or be provided with a wire prolongation to come close to the graduation; if the rectanglo is made inore open than is shown many turns of wire may bo used without biding the needle, or it may be fitted with an indicator, as in Fig. 44, and the graduation commence at right angles with the needle. Columns V. and VI. in the table present the action of exactly such an instrument—VI. being that of a single turn of No. 10 wire, V. that of the same continued with a number of turns of No. 18 fitted with a 3in. needle. It will be found, on trial, that these deflections bear no traceable relation to each other, though it is probably a combination'of the tangent and the sine of the angles varying at each point, and consequently they are nearly equal for the first 20° or 80°, in V. 4° being equivalent to 1° on the tangent instrument, i. е., 28° = 7°, and in VI. 1° equivalent to about 2,\°, but after this the value of the degrees increases in a much lower rate than in the tangent galvanometer.
162.—For many purposes it is common to arrange the needle in a vertical plane mounted on a central pivot, in which case the needle is double, one working inside a coil, as in Fig. 44, tho other with its poles reversed working outside. Such an instrument—which is, in fact, the needle telegraph instrument—has its uses, but its indications cannot be relied on; they vary with the varying magnetism of the needles, because the resistance to motion is not the magnetism of the earth but the extra weight of the lower parts of the needles; the chief advantage is their instantaneous action, as the needle does not vibrate as in the horizontal form, and their ready visibility from a distance. Col. VII. gives the action of such a vertical instrument made to form part of the circuit of a plating cell, as it has very little resistance of its own, and of course is able to be placed in anv position and glanced at at any time to see that all is right. The table shows one of the evils of this form in the sudden jump from 19° to 28'; this is due to some little irregularity in the friction of the mounting, or, perhaps, a slight bend in the wire on which the needles are
mounted, defects almost inevitable, but which render the instrument of little value for real measurement.
163.—When very feeble currents are to be measured, the resistance of the magnetic needle is reduced by rendering it nearly astatic, that is, without tendency to fixed position; this may be accomplished by placing above or below, at the proper distance, a bar-magnet whose action is opposite to that of the earth, so that the two nearly neutralize each other; it is, however, more commonly done by securing two similar magnetic needles on a wire at right angles ;■■ them, with their poles in opposite directions; the nearer equal the two needles are the Less the reresistance, and one of the needles working within the galvanometer coil aud the other outside the whole of the reactions tend to turn the combined needles in one direction. Such an arrangement it is however almost impossible to make perfect, and it will, therefore, always deflect farther to one side than the other with equal currents; also the actions continually vary with the altering magnetic conditions, and therefore it is an indicator rather than an actual measurer of currents.
164.—To obtain the most perfect results from a galvanometer its resistance should be about equal to that of the rest of the circuit, and, therefore, instruments with short thick wire, or with great leugths of very fine wire, either separate or combised in one, are usually employed. There is, however, a plan by which a galvanometer may be made widely available—viz., by providing means for sending the whole or part only of the current through the wire, the rest going through "derived circuits ;" in this case the instrument is made with a considerable length of fine wire suitable to the examination of а feeble current; if a German-silver wire of exact Jy the same resistance be now arrauged in the stand, so that by the insertion of a metallic p.'ug, or by any convenient commutator, it may form a complete connection or path for the current, this will divide itself equally through these two roads, and the indication of the galvanometer must be doubled because it is due to only half the current; by several such wires only a tenth or a hundredth of the current may be made to act on the needle, and thus the instrument may be made available for strong currents. There will, howevtr, always be a source of error in the different heating effects and consequent changes of conductivity, which is the reason why German-silver wire should be used, as its conductivity changes less with the temperature than that of simple metals. (To be continued.)
THE WORLD: IT3 FORMATION AND
Вт. Arthur Ukderhill.
I Continued /ro.-n paye 551.)
ABOVE the Silurian, aud below that most valuable and most interesting of the geological groups, the Carboniferous, lies a series of strata commonly known as the Devonian or Old Red Sandstone system.
As the name implies this group is eminently sandy (arenaceous), consisting pi incipally of sandstones, sandy shales, and schists, and a species of limestone, called cornstoue. from the fact of its being a kind of calcareous conglomerate. The ruddy appearance which more or less pervade« the greater portion of the group is caused by the presence of peroxide of iron, which, formed by the action of the water upon the iron veins of the previous epoch, was held in aqueous suspension, and finally deposited, together with the detritus, in the form of a now system. The Devonian group, as usually met with, consists of the following strata.
1. Yellow eandstonce and shales, with fossils of aquatic and land plants and fishes.
2. Limestones and schists, with shells and remains of crustaceans.
3. Coarse ruddy conglomerates and red sandstones, with plants and remaine of fish.
4. Red sandstones and conglomerates, green shale, aud limestone (cornstone); few fossils.
5. Rusty sandstones and conglomerates with flagstones and shales; few remains.
C. Dark flagstones and schists, with remains of molluscs, fish, and crustaceans, also of plants aquatic and terrestrial.
The physical features of the system show that it was for the most part formed from the deposits of a shallow tidal sea, the ripple-mark being very well and clearly exhibited. The fossils are principally marine, and in the case of animated beings solely so. Fuel, or seaweeds, and sedgelike plants (juncites) were numerous in the ocean of that day, and the hu<*e troe-ferns (adiantites), now inseparable from a ■warm climate, then flourished throughout the world. Among the animal kingdoms, the fishes reigned supreme j ichthyolites (ichthys, a fish ; and liihos, n stone), as the organic remains of fishes are cilled, exhibited in this period a far greater defrree of perfection than those of the preceding epoch. We find within this time-honoured tomb of solid rock the remains of these scaly inhabitants of the seas of innumerable ages ago, who, overtaken by the inexorable fate which awaits all living creatures, have been gradually buried to the depth of ten thousand feet. Among these '' ancient mariners '' are to be seen several curious creatures, of which the Cepltalospis (shieldheaded), Fig. 1, the Pterichthys (winged fish),
By The Rev. E. Kernan, Clongowes College.
(Continued from page 557.)
or more separately complete ships, joined together, and driven by the propeller of the last, Fig. 102.
somewhat resembling a tortoise with a tail, and the Diplacanthus gracilis, Fig. 2 (graceful doubleNpine), are specimens. One peculiarity is remarkable in the fish of this period, viz., the shape of their tails, which are all of a heterocercal form as it is called, that is to say, they were not symmetrical. Thus in Fig. 2 the tail is heterocercal, but that m Fig. 3 is termed homocercal, or symmetrical. The present race of fishes are nearly without exception homocercs, thus showing an entire change in marino animals. The above are of course merely specimens of the denizens of the sea who flourished in the waters of this epoch and of which the total number of genera was twenty-six, which were all new creations, and of which thirteen genera were peculiar to this system and perished upon its termination * great quantity of molluscs and radiates were also in existence, many being new creations, and some quite confined to and distinctive of this group
ine temperature of the globe was then uniform, the same fossils being found in the -Devonian strata wherever it occurs. In the now frigid plains of Russia and the warm and balmv moors of Devon, the same old plants, the same remains of then existing races of animated life appear and the tree-fern flourished as uxuriantly in the frigid zone as it does now in the torrid regions of the tropics. The chief cause
tntir^°rT£ Which attended this Peri°° «as !nf;rTP °n °.f Tr»PPean rocks, which appear in
Stn'l,?0?;n With the 01d Red Sandstone
. nation, in thfl form of greenstones and felspar
Sfc8' ^t^ gTMUe fa found howe,}er
had bvffifF fr°m tU8 that the &*aitic Period , the latter? tlme PTM* *? to the Trappean. as' thei Utter has smce done in favour of the volcanic.
2 l„,irrPtl-nS ««"M««My TMry the aspect of tlie locahties in which the system occurs, but, as a rule, it is somewhat tame, yet here and there
beauty. The appearance of the land durin- the Devonian epoch must have been of a verdantTcha imtstct°r?/fd ^^/^ftsof thegi-aceful and majestic tree-fern and other now tropical plants
"v the Z^ aD/-8ilent 8hadeS' undis'turbed even 'J»? m ?f msecta or the warbling of the ;»"5«t T?I**TM 7ho now l*0^ eTM>7 grove ouuSes^ l^ ^UdeQt mUSin- °» tte «**» °f vXl 11 °ted fnturies old> a delicious yet wM calm awakened only by the quiet rustling f the fern-leaves in the forest or the dull roar of
THE Bubject of the screw propeller may be
The "length" is, therefore, measured by a
The utility of the propeller is shown bv mentioning its advantages over other means o'f motion. For all sorts of ships its machinery takes less room, and can be placed lower down in the "hold." The shape required for quick sailing vessels is not spoiled; the propeller maybe used only as an auxiliary in calms, &c. When not acting it can be removed or otherwise prevented from injuring the speed. For merchantships the machinery may be in the "stern," leaving ample room for cargo. For war-ships the propeller is out of sight, can with great difficulty be injured by shot; much more space, too, is left clear for the armament.
As regards the question of " speed" only, the screw has not yet attained to what the paddlewheel can effect.
Though not very specially belonging to the propeller, this seems the best place to introduce a curiosity which appeared some time ago in the Mersoy,—the " jointed ship." The object of this ship is to avoid the waste and expense consequent on the multiplied machinery of several vessels lying idle during the discharge of cargo. For
When this compound ship has arrived at its discharging wharf, the machinery part is detached and goes off to seek " another train." Or it may happen that the porta of the "connector" have different destinations to which they are duly conveyed before the engine leaves. By this means one ship with a propeller does the work of several ships, as regards the moving power.
The history'fii the propeller is quickly told. Now that it haB proved such a success there are many laying claim to the invention. It is said that the propeller has been used in China for ages, and during the last century suggestions of it are to be found. The following facts are certain. In 1802, a Dr. Shorter used a screw to propel a boat. The same idea was again brought forward in 1813, by Mi-. S. Brown, C.E., London,* who in 1825 had a large ship moved by a screw. There are strong claims made for Mr. John 8wan, but the date of his invention, 1824, seems to decide against him. These facts were not much noticed for some years—the paddle-wheel absorbed all attention, when steam was applied to ships,— and it was not till 1837 that a real practical propeller was brought f orward. In that year Ericsson, a Swedish engineer, and Smith, an English amateur, had a small steamer built, 45ft. long, which was propelled by a screw. The success of this little ship was perfect. Notwithstanding, the Lords of the Adniirolty refused all encouragement to Ericsson. In 1839 a second screwsteamer was built, which also proved a success; but still the Board of Admiralty declining his offer, Ericsson left England in disgust.and followed his second vessel to America, there to develop his invention. The next year (1840), Smith and others ventured on a larger scale; the Archimedes, 232 tons and 80-horse power, proved beyond all i doubt the practical efficiency of the screw as a i propeller. The success of this vessel, at length decided the Admiralty, and they ordered a vessel to be built, the Rattler, with the propeller of which the shortening experiments were carried. From this date the screw came more and more into favour, and it is now to bo found in every class of ship, from the largest man-of-war, oceansteamer, or sailing vessel, to the smallest pleasure yacht.
Sciiew App. V.—Aerial Navigation.—Some years ago (1863), a company was formed in Paris to carry out the ideas laid before a brilliant assembly by M. Nadar, who expressed himself most confident that now indeed had been discovered the true means of directing a body moving in the air.t The principle on which the new discovery depended was, that to be directed, a body should be lieavier than the medium in whioh it floats. Rejecting the balloon therefore as quite unfit, the inventor proposed to raise the vehicle into the air by means of screw-propellers. The scientific toy, four vanes, Fig. 103, spun by a cord (a), or spring (*)
« All rights reserved.
rising mto the air, was the point de depart of the auto - locomotion aerienne, of M. Lalandelle. On a very small scale successful models were shown to the assembly; nothing, however, practically useful has come before the public as vet.
Sub. App. III.—Thi Leech Propeller. — Some years ago, under the above title, a propeller was invented, the action of which was to be that of the leech when swimming. The propeller was a flat pointed" band," extending, in a hollow
* English Mechanic, Vol. IV., p. 220.
chamber, from stem to stern of the ship. An undulating motion, communicated to the " baud," produced a series of inclined planes, which incessantly changing position, would propel the ship at в тегу high speed. This invention seems not to nave succeeded.
Pboblems.—From the great length to which the Applications have extended, the student cannot but be impressed with the importance of the law s of forces " applied to a point," and may have been anxiously wishing for some exercises by which to fix them in the mind. Problems could not well have been introduced—at least, in a book —up and down through the Applications; and Dow even so much space and time has been taken up with the mere necessary development of principles, that a good long series of problems, such as ought to be given, would be, perhaps, too great a delay. A few, therefore, just to recall the general principles of this section, and to put those who may wish it in the way of explaining the exercises abundantly provided in books on statics.
Questions On The Laws Op Оггозгге Forcer.— Рвов. I.—A ship is held safely in ordinary circumstances of wind, «fee, by a single anchor. Explain—by " forces to a point," and " opposite."
Prob. II.—Why is it necessary sometimes in great storms to throw out a second, a third anchor, and even to get up stoam to work ahead (paddle or screw) slightly?
Рвов. III.—A suitable anchor is able to resist the drag of the largest ship. How then is it possible ever to draw up the anchor firmly held below, and is there not sometimes danger of the chain breaking?
Рвов. IV.—A net is to be fixed in the narrow side passage A of a river (Fig. 104) tho current
of which has a force on the net of five tons. A «mall bank, B, terminating a shallow in the river, »ill just safely bear one ton of stones, in a box; л rope from which to the net makes an angle of 12° with the direction of the current. Required, the direction and intensity of a force, Q, on the bank sufficient to keep the net in position.
Prob. IV.—A largo lamp, weighing 25 cwt., is to be hung midway between two rocks at either side of a wide river. The top of the rocks form an angle of 100" with the spot where the lamp should be. Required, the necessary strength of the two ropes which will hold tho lamp—no question of winds, Ac.
Prob. V.—Why does a ship held by a single anchor swing round with the wind, tide, &c, and hold a direction exactly opposite to any one of these, if that one be ahme in action? What would be the direction more or Use, if a strong current cross the wind at an anglo of 50°? and in this latter case, what element should be given to make it possible to have an exact answer?
Prob. VI.—Has a horse, drawing a canal boat, more power with a short or long rope, the weight «f the rope not being taken into account? Would ; s much power be required in the boat—suppose a propeller to be used—as is exerted by the horse from the bank?
Questions On The Inclined Plane.—Prod. I. —A block 50 tons weight, length of plane 500ft., height 30ft. Required the power necessary for equilibrium, when the rope may be disposed parallel to the length. Show how the principle of " virtual velocities " is here exemplified.
Рвов. II.—A ship 500 tons weight is to be drawn up an inclined plane 100ft. long, 20ft. high. The power at command is only equal to about 50 tons, and cannot be increased. Still the ship must be raised without being uuladcn. What is to be done?
Рвов. III.—A single storied house, Fig. 105. is built upon the side of a hill, and tho foundations, А В dotted lines, are slanted as the hill. The house having stood solidly for years, two stories (according to the original plan) are added to it. Is this house now in any danger, and why? (.'an any precautions be taken against danger?
Prob. IV.—Wedges of wood driven into stone split the stone when they have been wetted. Is this a wedge action or not ? and why? Show in diagram the direction of the forces.
THE SWARMING OF BEES AND THEIR METAMOEPHOSES.
ГГ^НЕ question has often been asked, Is bee-keepJ_ ing profitable '.' and numbers are endeavouring to solve the problem by practical experience. Many cottagers now miugle pleasure with profit and keep a hive or so of bees, and even in Loudon these useful and industrious insects arc often to be seen in the neighbourhood of gardens. Costing but little for food, and making use of almost anything for a hive, they demand only slight attention, and are thus well adapted as an amnsing recreation for the labouring population. There is one point connected with bee management which has always been a subject of dispute—viz., the method of swarming, and whether swarming or non-swarm ing bees are the more profitable. The latter point would seem to be settled, as we find a writer saying:—After making many trials we can state that in good seasons for honey, a good early swarm will, at the end of the season, weigh more than a hive that has never been permitted to
swarm at all. A swarm put into an empty hive is doubtless placed at a great disadvantage, and apparently will never both fill its hive with combs and gather as much honey as the old one already full, weighing perhaps 301b. or 401b. But wait a little; the swarm that is far behind during the firs: t«n days, afterwards rapidly gains upon the old оье. and generally overtakes it when they are both about 701b. or ROlb. each; the young one now g.)*- ahead, at the rate of 21b. for lib. And, besides tl.r i/roat superiority of the first swarm over the hi-,■< which did not swarm, there are the mother hive and probably a second swarm, weighing by the end ol the M us,,n from 401b. to 801b. each.
The author accounts for this by%aying that th-. stimulus of an empty hive makes the bees wort harder. He goes on to say that in the non-swarming mode of management the queens become old Sjiu die ; and at the time ef the death of a queen therv is a loss sustained. The hive in which she dies is without eggs for three weeks, or thereabouts; for lUnarilythe young queens are not matured til! about ten days after the old one dies, and it is ttc days more before she begins to lay. But there is the risk of the loss of the whole, for if the quetii dies when she is not laying, the bees cannot raise I successor.
A writer in the Maine Farmer (U.S.), thus dt scribes a successful method of swarming bees ;—
"First, for qneeu rearing I select from a stori noted for its superior activity and working qualities, two or three frames, one or two of a brood ix the proper stage, and one of honey, with enougt lues to maintain the necessary degree of heat. Я iiich I phice in an empty hive and cover wiii. flannel, which retains the heat. In this nucleus I had usually six to ten perfect queen cells, if I lmv■ nough bees and plenty of brood in the right stage \l>out the eighth <lay Ï take from as many stocks. less one, as I have cells, liées, honey, and brood, and form other nuclei, one for each ceil, which I insert carefully in each, being sure to put the cell in the centre of the brood, tliat it may be discovered and protected. Let these second nuclei stand twenty-four hours before the cell» are inserted, else tho bees, unconscious of their loss, may aestrov them. This I had to learn by experience.
"To make my new swarms, I divide my bees and combs as near as possible, one-third of the bees in one hive and two-thirds in another, which last 1 place upon a new stand, but the combs equally. By using the queeu and bees that were taken from the same hive and divided, it is not necessary to cage the queeu or use other methods which may disturb the day's work, and by isiug a fertilized queen nothing is lost by waiting to rear one. I never divide without either I queeu, or, in default o/ one, a capped cell, which saves much valuable time."
As regards the various metamorphoses of bees, some careful observations have recently been niatl«in .Switzerland according to which the development of queens, drones, and workers proceeds as follows, in the ordinary temperature of the hives in sprin x» and summer :—The egg hutches on the third day after being laid. The queen remains in the larvul stale in the open cell, five days, the worker tWe days, and the drone six days and twelvo hours. Iu spuming the cocoon, the queen spends one day. the worker one day and twelve hours, and the drone three days. After spinning the cocoon, the queeii remains a larva two days and sixteen hours, the worker three days, and the drone two days and twelve hours. After changing, the queen remains in the nymph or pupa- state four days and eight hours, the worker seven days and twelve huor?. and the drone nino days. Hence, from the capping of the cell to the issuing of the bee, the queen usually requires eight days, the worker twelve, and the drone fourteen days and twelve hours—making, from the laying of the eg^rs to the emerging of the perfect insect, the normal period of sixteen days forthe queen, twenty for the worker, and twenty-four for the drone. This period. however, is occasionally hastened er retarded by tVie peculiarly propitious or nnpropitioos state oí the weather or the temperature of the hive; and the term has been found to vary, the queen, from thr lath to the 22nd day; iu the worker; from, tfc lüth to the 26th ; and in the drone, from the -ХатЛ to the 28th day.
MANUFACTURE OF PORTLAND СЕМЕ XT.
JJOBTLAND cement was introduced to pubh notice under a patent by an EagtfattB— nearly fifty years ago; and we have hitherto pos sessed a partial monopoly in its production, inasmuch as we have fortunately inexhaustible beds of, the raw material from which it is made, and ecu abundant supply of fuel necessary for their economical manufacture. It is strango that ander these conditions French engineers should hsv. obtained the start of their professional Aw/raven in this country, and that they should have be^e thr first to demonstrate by experiments, and ыцЬи queutly by the erection of magnificent barbean works on their seaboard, the valuable properties at this excellent constructive material. W« may date the txtemiee employment of Portland wuu&ut in England from the commencement of the metropolitan main-drainage works. Daring the last fifteen years the manufacture of Portland cement has gone on steadily increasing, until at the present day we find that little short of -400,000 tons per annum are made h» the county of Kent—the centre of cement manufacture—irrespective of the productions of many nainoir factories in different parts of the country.
The chemistry of tie sotting of Portland cement is liy no means so well understood as it ought to-be. There is no doubt, however, that, like the hydraulic lime and natural cements, it in, chemically speaking, n doubk* silicate of lime and alumina; silicic acid is generated by the hydration of the cement, and forms insoluble salts with the lime and ahrmina bases. It is a curious fact that Portland cement hardens more ra-pidly when salt water is employed. According to Scliweitzer, 1,000 grains of sea-water in the English Channel contain 27060 grains of chloride of sodium; soluble silica has a Imowrt preference for alkaline banes, and it is not improbable, when the cement is hydrated with sea-water, that the chloride of sodium is decomposed, the silicic acid of the cement combining with the sodium and oxygen of the water, and forming thereby a silicate of soda, or a species of crude glass.
Portland cement is of two classes, which, ffor the sake of distinction, may be termed •' Engineers'" cement and " Plasterers'" cement. Tne former is the more costly; it is usually described by manufacturers as "best heavy tested;" it weighs from 1121b. to 1201b. to the bushel, is slow setting, and of great strength; the latter is a light cement, quick setting, and of inferior strength when compared with the other. It must be understood that mir remarks apply exclusively to "Engineers'" cement.
Portland cement is made from chalk and alluvial clay; the factories on the banks of the Thames use white chalk, those on the Medway grey chalk; the hitter is rirobably preferable, inasmuch as it contains large quantities of silicious matter. Mr. Read, in his treatise on "Portland Cement," says that "the present and safest proportions, provided both chalk and clay are selected free from sand, are four parts of chalk from the Medway (grey), or three parts of Thames (white), with one of clay by measure." These materials are placed in mills of simple construction, each having a circular pan, Sft. in diameter and 2ft. deep, in which two '• edge runners," 4ft. Bin. in diameter, are kept continually going; a constant stream of water flows into the pan, and as the "edge runners" revolve, the chalk and clay are thoroughly ground, and, being thus converted into a fluid state, they filter through a band of fine brass-wire gauze fixed to the side of the pan, and flow through wooden "hunters" into tanks or settling reservoirs. One washiiill will feed four tanks, each of which is about 100ft. long, 40ft. broad, and 4ft. deep. When one jf these has been filled in the manner just described he same process is applied to the others in succession. About three weeks after the tanks are filled he whole of the materials will be precipitated, the lear water being drained off in the mean time hrougli a small weir in the brick side of the tank; lie residuum is a plastic mixture of the consistency >f "putty," and not much unlike it in colour. The icxt process is to convey this precipitate from the lank to the " drying floors," over which it is spread in a layer abont Gin. thick; each floor is 40ft. by X ift.; it consists of an outer skin of boiler plates resting on a series of brick ovens and flues. The abject of this arrangement is to render the plates sufficiently hot to effect the rapid desiccation of Jie water from the superincumbent layer, a process .'eiierally accomplished in about twelve hours. L'he materials having thus been thoroughly !rie«l are ready for conveyance to the kilns, i'be "charge " consists of alternate layers of coke nd raw materials, the burning generally occupying hirty-six hours. When the contents of the kiln beotnes sufficiently cool, the " clinkers,'' or cement tones—for the mixture has now assumed that form -are drawn and removed to a floor where the arger pieces are broken, and the whole of the burnt mterinls are then conveyed to the hoppers o f the rrinding-mills, where, passing under rapidly revolvng horizontal burr-stoues, they are ground into an boost impalpable powder. The cement issues rom the mill at a temperature of about 160°, and he now manufactured material is wheeled away, nd spread in a layer from 2ft. to 3ft. thick over the uor of a cool shed; it is subsequently packed in isks< or sacks for conveyance from the works. The sseutial conditions for the manufacture of good ortland are: 1, The chalk and clay should be loroughly mixed in the wash-mills, and the fluid aterials delivered by "launders" over the entire -ea of the settling tanks. 2, The contents of the ilns ought to be burnt equally throughout. 8, be burnt materials should be ground very flue.
After coming from the mill the cement should be jread over the floor of a shed, and allowed to renin there for at least a fortnight previously to -ing packed into casks or sacks. The strength of Portland cement increases as its leciflc gravity increases; the tensile tests arc lUidly made with briquettes the standard size for
the neck being 14in. by 1 Jin.; and it must he understood that all experiments referred to have reference to the weight necessary to sever 2$ square inches of neat cement.
It appears from Mr. Grant's valuable paper, read before the Institution of Civil Engineers in December 1863, that Portland cement gains from 20 to 3ftpar cent, in strength by setting under water; it is usual, therefore, to place the test briquettes in water, after gauging, and to allow them to remain there until they are to be tested. The following table has been compiled from a recent series of experiments; it shows the average tensile strength of I 'unbind cement as compared with the natural cements; the test blocks were of standard size of 2 J squarwiuches, and placed in water as before described:—
The Builder*' Trade Circular vouches for the accuracy of these figures.
Mr. Grant's tables show conclusively that the strength of gauged Portland cement increases with age; from his experiments it appears that the breaking weight of rest blocks, one week old, one year old, and two years old, are as 1, 1'5, and 1G2. The ultimate maximnm tensile strength has not as yet been ascertained; experiments are, however, being conducted periodically with a view to determine this important point. Mr. Grant gives the average tensile strength of cement weighing 1191b; to the bushel as 7771b., whereas we give it as 10241b., the excess of the breaking weight as recorded by us may probably be accounted for by improved manufacture since Mr. Grant's experiments were made.
Portland cement now forms an important item in the list of our manufactures, but even now its valuable properties are not as fully appreciated as they deserve to be.
WROUGHT IRON CRANES.—in.
OUR two preceding articles on this subject were confined solely to a practical description and illustration of the ordinary method of constructing wrought-irou cranes, and we reserved for a third and concluding article the question of their theory and scientific analysis. It has been already stated that mathematical formula; and algebraical equations are not suitable for those particular examples of construction in which the different parts are inclined to one another at angles which not only vary among the parts themselves, but are also dissimilarly inclined to the horizontal. An example of our meaning is given in Fig. 2, which represents a skeleton elevation of a wroaght-iron
bent crane. In practice, the outer and inner flanges arc curved as shown by the dotted lines, but they are considered theoretically to consist of a series of straight lines joining the several points where they are connected with the diagonal bars forming the web. These points may, in reality, be either pins or rivets, but bearing in mind to what tremendous jerks and vibrations cranes are subjected, there would not be any particularly valid reason for employing the former kind of attachment. After a time, pins must work loose; rivets never do. To determine the strains upon the crane represented in Fig. 2, which is supposed to be loaded only with a weight at its peak L. we start from the fundamental principle that governs the distribution of strain upon two separate members inclined at different angle9 to the horizontal, and acted upon by the same weight—to speak dynamically, by the
same force. The principle, which it is necessary thoroughly to understand, will be best explained bjr a reference to Fig. 1. Let A C, B C, be two barf inclined to the horizontal line F G at angles respectively of 60° and 45°, and supporting a weight W at their junction C, which let us take^qual to 1 ton. By the method of resolution of Rfcee. construct the triangle C D E, and make C D = 1 tonDraw D E parallel to B C, then D E will equal the strain upon B C, and E C that upon AC. If these strains be measured off upon a scale of two tons to the inch, which is the scale npon which C D is drawn, they will be found to equal respectively 050 and 0-71. It is not difficult to prove that the v values lire in the ratio of the cosines of the angles which the bars moke with the horizontal line F ft. By the principles of trigonometry we obtain in the triangle C D E the proportion D E: E C : : sine E C D : sine EDC. But the angle E C D is the complement of the angle E C F, which in the diagram was made equal to 60 degrees; similarly, the angle EDC is equal to the angle DCB (Euclid, prop, xv., book 1). which is the complement of the angle B C G, wlueh is equal to 4fi degrees. As the sine of any angle is equal to the cosine of its complement, if we call the strain upon the bar BC = ED = S, and that upon A C = E C = 81, we have S : S' : : cosine 60" : cosine 45°. Substituting the values of these in the equation, we find 05 : 071 : : 0-50000 : 0-70710, a sufficiently close approximation to prove tho accuracy of the diagram.
A similar method is employed in deducing the strains upon the crone in Fig. 2, with the exception that, as we proceed, the resolution of the different forces becomes rather more complicated, but not to such a degree as to hinder any one from mastering the subject with care and attention. At the some time it is not to be supposed that a mere casual perusal of our article is sufficient to enable our younger readers to comprehend the matter thoroughly. Far from it. They must study—not merely read—it, and work out for themselves the diagrams to a conveniently Urge scale. For the sake of simplicity, we will suppose that there is a weight of one ton only at the point. L, in Fig. 2 The advantage to be derived from this is that when once the strains dne to a weight of one ton have been ascertained, all that is necessary to obtain those on another crane similar in design but loaded with a different weight is to multiply them by the constant ratio between the original or standard weight and the one in question. It must not be forgotten that any alteration in the design of the crane, such as changing the radii of either the inner or outer flanges, or the angle of inclination of the bars, will altogether vitiate the calculation. To return to our diagram in Fig. 2. Let L a equal one ton, supported at the front L. How does this produce strains differing in character and intensity throughout the whole structure? In the first place. the weight being supported at the connection of the two bars K L and L H must strain both of them, compressing the latter and extending the former. The parts in compression are shown by thick black hues, those in tension by fine ones. From the point a, draw a b parallel to the bar K L, meeting the chord of the part L H of the lower flange in 6. Then, measured on the Bame scale, a b will give the strain upon L K, and L // that upon L H, and they are respectively equal to — 1-3 and + 1-05 tons. The sign minus denotes a tensile strain and -f- s compressive. Leaving this latter strain alone for a moment, let us trace the further action of that upon L E. This is obviously transferred to point K, where it acts upon G E and E H, extending the former, and compressing the latter. Upon K L. plot off Kc equal to a b, equal to the strain upon K L; draw e d parallel to K G, meeting K H in </. The lines c d and E d will give the strains upon G K and E H respectively, and will be found equal to — 2'1S and + 1-86 tons. Following the strain upon K H, it is transferred to the point H, where it pulls upon the bar G H and compresses H F. If there were no other force acting upon the point H, nothing more would be required to find the strains upon the members G H and H F than to proceed as before. But there is the strain upon L H, equal to L b, to be taken into consideration, and how it also affects the bars acted npon by the strain upon E H. It will be seen on inspecting the diagram that it compresses H F, and also compresses the bar G H, which is, on the contrary, stretched by the strain brought upon it from E H. But it has been frequently mentioned that when a bar or part of a structure is acted upon by forces or strains of both tension and compression, the actual strain is the algebraical sum of the two. Thus if a bar have a tensile or minus strain of 7 tons, and a compressive or plus strain of 4 tons, then the actual resulting strain is (+ 4 — 7) = — 3 tons. This is just what occurs in the present cose, and we thus see h»w important it is to remember these fundamental axioms. The bar G H is acted on by a tensile strain in the direction of E H and a compressive one in that of L H. It is the difference, therefore, of these two which is the strain really affecting the bar.
There are two methods of resolution by wliich these strains on G H and H F may be determined.
One is by proceeding as has been already done, taking care to take the algebraical sums of strains of a different character where they are so, and the other to first find the resultant of the two original strains, and then completing the diagram of forces. Let us consider the former method first. Produce К H to e, making He = E d = strain upon E H. From « draw e /, parallel to G H. Measure « / and and/H, and these will represent the strains upon the bars G H and H F, due to that transferred to the point H by the bar E H, and will be - 10 and + 2-4. Now for the other strains, produce L H to I, making H I equal L b equal the strain upon L H. From I draw I n parallel to Ci H, and I n and n II will be the other strains upon G H and H F, and will scale + 05 and + 063 tons. Summing up, we obtain the total strains upon these parts of the crane to be as follows:—Upon UH (-10 + Oil = - 0-5 tons, and upon H F (+ 84 + <И») = + 303 tons. By the second method, the resultant of the strains of К H and L H must be first ascertained. Produce E H to e. as before, draw e g parallel to L H, and equal to L h. From g draw 9 A, parallel to G H. Then g It will be found, on measurement, to equal le f — I n) = — 05 and h H to eqnal (H » + Hf) = + 3-03 tons. At the point C, there are also two forces acting—namely, the strain due to the bar G H and that to K G. The resulting strains can easily be determined upon the parts G E and G F by either of the methods we have just investigated. We cannot spare space to go through every strain, nor is there any necessity, as the determination of the others is simply a repetition of what has been already done. Those who desire to thoroughly understand this operation must make a good large diagram of Fig. 2, and work out the whole problem for themselves, taking care to tabulate the strains upon each bar separately, so as to perceive how they accumulate from the free to the fixed extremity of the crane.
Although this method of calculating strains is perfectly accurate enough for all practical purposes.
(Concluded from page 536.)
Aneroid gauge, known as the " Bourdon gauge,'' from the name of its inventor, a Frenchman. B is a bent tube closed at its ends, secured at C, the middle of its length, and having its ends free. Pressure of steam or other fluid admitted to tube tends to straighten it more or less, according to its intensity. The ends of tube are connected with a toothed sector-piece gearing, with a pinion on the spindle of a pointer which indicates the pressure on a dial.
300. Pressure gauge now most commonly used. Sometimes known as the •' Magdeburg gauge," from the name of the place where first manufactured. Face view and section. The fluid whose pressure is to be measured acts upon a circular metal disc, A, generally corrugated, and the deflection of the disc under the pressure gives motion to a toothed sector, e, which gears with a pinion on the spindle of the pointer.
301. Mercurial barometer. Longer leg of bent tube, against which is marked the scale of inches, is closed at top, and shorter one is open to the atmosphere, or merely covered with some porous material. Column of mercury in longer leg, from which the air has been extracted, is held up by the pressure of air on the surface of that in the shorter leg, and rises or falls as the pressure of the atmosphere varies. The old-fashioned weather-glass is composed of a similar tube attached to the back of a dial, and a float inserted into the shorter leg of the tube, and geared by a rack and pinion, or cord and pulley, with the spindle of the pointer.
302. An " epicyclic train." Any train of gearing the axes of the wheels of which revolve around a common centre is properly known by this name. The wheel at one end of such a train, if not those at both ends, is always concentric with the revolving frame. C) is the frame or train-bearing arm. The centre wheel, A, concentric with this frame
yet, as it is a consecutive one, it is advisable to ! gears with a pinion, F, to the same axle with which check the strains in the flanges at intervals. This \ • ...... '*"
may be readily done by very simple formula?. For example, the strain upon the bar E G has been found by the diagram to be equal to — 215 tons, and it may be checked as follows. It is manifest from the principle of moments that the strain at any point multiplied by the leverage with which it acts, will be equal to the weight at that point, multiplied also by the leverage with which it acts. The strain on the bar G E acts with a maximum leverage of N H, equal to 35 feet, whereas the weight at Lacts with the horizontal leverage of H a, equal to 79 feet. If S be the strain required, L its leverage, W the weight, and L' its leverage in the general for
„ W + L' 1 + 79 muía, S = т — ».с • From which we
obtain S = - 2-2 tons, which checks the accuracy of the calculation by diagram. Great care must always be taken in calculating by diagram, as there is always a tendency for errors to accumulate. One false step at the commencement, if it is not perceived, is perpetuated in all the succeeding operations, and would ultimately cause a very serious difference in the correct values. But by using th<\ method of moments to check the results, no such accumulation can occur, and one is enabled to perceive, as the working out of the diagram proceeds, that the calculations are rightly performed. One or two checks of this kind should always be introduced, as, if the whole operation is effected first, and the strain upon the last flange not found to be correct, the whole work must be gone over again to discover the error.—Building Лгеюа.
is secured a wheel, E, that gears with a wheel, B H the first wheel, A, be fixed and a motion be given to the frame, C, the train will revolve around the fixed wheel and the relative motion of the frame to the fixed wheel will communicate through the train a rotary motion to B on its axis. Or the first wheel as well as the frame may be made to revolve with different velocities, with tho same result except as to the velocity of rotation of B upon its axis.
In the epicyclic train as thus described only the wheel at one extremity is concentric with the revolving frame ; but if the wheel, E, instead of gearing with B, be made to gear with the wheel, D. which like the wheel, A, is concentric with the frame, we have an epicyclic train of which the wheels at both extremities are concentric with the frame. In this train we may either communicate the driving motion to the arm and one extreme wheel, in order to produce an aggregate rotation of the other extreme wheel, or metion may be given to the two extreme wheels, A and D, of the train, and the aggregate motion will thus be communicated to the arm.
303. A very simple term of the epicyclic train, in which F, G, is the arm, secured to the central shaft, A, upon which are loosely fitted the bevelwheels, C, D. The arm is formed into an axle for the bevel-wheel, B, which is fitted to turn freely upon it. Motion may be given to the two wheels, C, D, in order to produce aggregate motion of the arm, or else to the arm and one of said wheels in I order to produce aggregate motion of the other I wheel.
304. "Ferguson's mechanical paradox," designed to show a curious property of the epicyclic train. The wheel, A, is fixed upon a stationary stud about which the arm, C, D, revolves. In this arm are two pins, M, N, upon one of which is fitted loosely a thick wheel, B, gearing with A, and upon the other are three loose wheels, E, F, G, all gearing with B. When the arm, C, D, is turned round on the stud, motion is given to the three wheels, E, F, G, on their common axis, viz., the pin. N ; the three forming with the intermediate wheel, B, and the wheel. A, three distinct epicyclic trains. Suppose A to have twenty teeth, F twenty, E twenty-one, and G nineteen; as the arm, E, C.D.ia turned round, F will appear not to turn on its axis, as appoint in its circumference will always point m une direction, while E will appear to turn slowly in one and G in the other direction, which—an apparent paradox—gave rise to the name of the apparatus.
305. Another simple form of the epicyclic train, in which the arm, D, carries a pinion, B, which gears both with a spur-wheel, A, and an annular wheel, C, both concentric with the axis of the arm. Either of the wheels, A, C, may be stationary, and the revolution of the arm and pinion will give motion to the other wheel.
30«. Another epicyclic train in which neither the first nor last wheel is fixed, m, «.'is a shaft to which is firmly secured the train-bearing arm. £, I, which carries the two wheels, d, e, secured together, but rotating upon the arm itself. The wheels, b and t' are united and turn together, freely upon the shaft, m, n; the wheels,/and g, are also secured together, but turn together freely on the shaft, n. n The wheels, r, d, e and/, constitute an epicyclic train of which C is the first and/ the last wheel. A shaft, A, is employed as a driver, and has firmly secured to it two wheels, a and n, the first of which gears with the wheel, b, and thus communicates motion to the first wheel, c, of the epicyclio train, and the wheel, A, drives the wheel, g, which thus gives motion to the last wheel). Motion communicated in this way to the two ends of the traui produces an aggregate motion of the arm, t, I, and shaft. m ». This train may be modified: for instance, suppose the wheels, g and/, to be disunited, g to be fixed to the shaft, », в, and/only running loose upon it. The driving-shaft, A, will as before communicate motion to the first wheel, c, of the epicyclic train Irr means of the wheels, a and b, and will also "7" cause the wheel, g, the shaft, m, n, and the trambearing arm, i, I, to revolve, and the aggregate rotation will be given to the loose wheel).
307. Another form of epicyclic train designed for producing a very slow motion. m is » fixed enatt lipon which is loosely fitted a long sleeve, to the lower end of which is fixed a wheel, D, and to the upper end a wheel, E. Upon this long sleeve there is fitted a shorter one which carries at its extremities the wheels, A and H. A wheel, C, gears with both D and A, and B train-bearing arm, », », which revolves freely upon the shaft, m, p. carries upon a stud at » the united wheels, F and G. If A have 10 teeth, C 100, D 10, E 61, F 49, G 41, and H 51 there will be 25,000 revolutions of the train-bearing arm. m, », for one of the wheel, C.
308. A better form of the " Lewis " than that given in fig. 293, for which we are indebted to our correspondent " Theta."
Messbs. Baïïeb Asd Емиквгно are investigating the nature of Indigo with a view to its artincial preparation. Their researches are apparently sngtîeatcd by uie success of Messrs. Graebe and Liebermaun with respe« to Alizarine.