evident that the force of the current is proportional to the sine 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. But, 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.

163. When very feeble currents are to be measured, the resistance of the magnetic needs is reduced by rendering it nearly astatic, that a without tendency to fixed position; this may accomplished by placing above or below, at proper distance, a bar-magnet whose action opposite to that of the earth, so that the nearly neutralize each other; it is, how more commonly done by securing two magnetic needles on a wire at right ang them, with their poles in opposite directions nearer equal the two needles are the less resistance, and one of the needles working the galvanometer coil and the other outs whole of the reactions tend to turn the com needles in one direction. Such an arrang it is however almost impossible to make p and it will, therefore, always deflect furthe one side than the other with equal currents; the actions continually vary with the dig magnetic conditions, and therefore it is a cator rather than an actual measurer of c

tion. It is, however, quite easy to obtain the mounted, defects almost inevitable, but which
real value of their deflections, by once carefully render the instrument of little value for real
comparing them with a sine or tangent instru- measurement.
ment. The process is to connect one of the
screws with the positive pole of a constant bat-
tery, such as a Daniel or Grove, the other
being connected to one screw of the other gal-
vanometer and through it to the negative pole of
the 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-
ment to be tested is entered opposite to that shown
by the standard instrument; by increasing the
length of the connecting wires and using fine Ger-
man-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 deflec-
tions of the instrument, providing the same
needle be always used with it. For convenience
it is best to note upon the face of each galvano-
meter the side to which a 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

FIG. 45

form. A is the stand mounted on levelling screws
a; 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 de-
scribed, section 156, in the tangent galvanometer,
so as to divide the degrees; in the centre is a
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 c is a handle by which to move it.
On it is fixed C, a wooden frame on which is be let into the wood frame, or even be on the
wound the wire; ns is the needle, which in this
instrument should be heavy and highly mag-
netized 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 indi-
cator with a line marked on its middle; e is a
point fixed to the stand on the line of 90 degrees;
+ and are 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 re-
main fixed, and communicating by means of

springs pressing on metallic bands on B, which would convey the current to the wire; but these 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.

lower side, so as to allow the graduated are to
be on the wood, and the needle should either
curve down or be provided with a wire prolonga-
tion to come close to the graduation; if the rect-
angle is made more open than is shown many
turns of wire may be used without hiding 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 de-

=

164. To obtain the most perfect result a galvanometer its resistance should be s equal to that of the rest of the circuit, therefore, instruments with short thick wire, e with great lengths of very fine wire, either sepe rate or combined 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 a feeble current; if a German-silver wire of exactly the same resistance be now arranged in the stand, so that by the insertion of a metallic plug, 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, however, 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.)

As the name implies this group is eminently sandy (arenaceous), consisting principally of sandstones, sandy shales, and schists, and a species of limestone, called cornstone, from the fact of its being a kind of calcareous conglomerate. The ruddy appearance which more or less pervades 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 new system. The Devonian group, as usually met with, consists of the following strata.

BOVE the Silurian, and below that most flections bear no traceable relation to each other, A though it is probably a combination of the tangent logical groups, the Carboniferous, lies a series valuable and most interesting of the geoand the sine of the angles varying at each point, of strata commonly known as the Devonian or and consequently they are nearly equal for the Old Red Sandstone system. first 20° or 30°, in V. 4° being equivalent to 1° on the tangent instrument, i. e., 28° 7°, and in VI. 1° equivalent to about 24°, 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 arTo use the instrument it is placed in the mag-range the needle in a vertical plane mounted on netic meridian with e pointing truly to the line on a central pivot, in which case the needle is d, and the zero or 0 on the circle at the middle double, one working inside a coil, as in Fig. 44, line of b, or if not, then the deviation is to be The connoted and applied to the final result. nection is now made, and B turned round till a again rests opposite e; the angle through which B has been moved gives by its sine the proportionate value of the current, and by constructing a table and making 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, and the instrument too powerful, and hence the necessity for the power to use only one or more turns of the wire, for each of which lengths a special column will be needed in the table for the actual value based on experiment with that length.

160.-Ordinary galvanometers are of more simple construction, but give no definite informa

the other with its poles reversed working outside.
Such an instrument-which is, in fact, the needle
telegraph instrument has its uses, but its indi-
cations 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 instan-
taneous action, as the needle does not vibrate as
in the horizontal form, and their ready visibility
Col. VII. gives the action of
from a distance.
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 any 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

UPPER SERIES.

1. Yellow sandstones and shales, with fossils of aquatic and land plants and fishes. 2. Limestones and schists, with shells and remains of crustaceans.

MIDDLE SERIES.

3. Coarse ruddy conglomerates and red sandstones, with plants and remains of fish.

4. Red sandstones and conglomerates, green shale, and limestone (cornstone); few fossils.

LOWER SERIES.

5. Rusty sandstones and conglomerates with flagstones and shales; few remains.

SEPT. 9, 1870.]

ENGLISH MECHANIC AND MIRROR OF SCIENCE.

6. Dark flagstones and schists, with remains of molluscs, fish, and crustaceans, also of plants aquatic and terrestrial.

the surf upon the beach. Such a prospect forcibly
calls to mind that beautiful passage in the work
of the younger Pliny:-

O mare! O littus verum secretumque Moveelov! quam
multa invenitis! quam multa dictatis!

The physical features of the system show that it was for the most part formed from Thou solemn sea and solitary shore, best and most the deposits of a shallow tidal sea, the retired scene for contemplation, with how many noble ripple-mark being very well and clearly ex- thoughts have ye inspired me! hibited. The fossils are principally marine, and in the case of animated beings solely so. Fuci, or seaweeds, and sedgelike plants (juncites) were numerous in the ocean of that day, and the huge tree-ferns (adiantites), now inseparable from a

(To be continued.)

ELEMENTARY SCIENCE."

(Continued from page 557.)

APPLICATION X.

INCLINED PLANE.

warm climate, then flourished throughout the world. BY THE REV. E. KERNAN, CLONGOWES COLLEGE.
Among the animal kingdoms, the fishes reigned
supreme; ichthyolites (ichthys, a fish; and lithos,
a stone), as the organic remains of fishes are
called, exhibited in this period a far greater de-
gree of perfection than those of the preceding
epoch. We find within this time-honoured tomb
of solid rock the remains of these scaly inhabi-
tants 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 Cephalaspis (shield-
headed), Fig. 1, the Pterichthys (winged fish),

FIG.I

FIG. 2

FIG. 3

somewhat resembling a tortoise with a tail, and the Diplacanthus gracilis, Fig. 2 (graceful doublespine), 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 in Fig. 3 is termed homocercal, or symmetrical. The present race of fishes are nearly without exception homoceres, thus showing an entire change in marine 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. A great quantity of molluscs and radiates were also in existence, many being new creations, and some quite confined to and distinctive of this group.

The

THE subject of the screw propeller may be
Tclosed by a few words on the technicalities
of the propeller, its utility, its history.
"technicalities" contain a number of terms of
every-day use which should be familiar. Pitch
of the propeller, is the "pitch" of the screw
(the height of the plane) of which it (the propellor)
is a portion. The Great Eastern, for example,
37ft. pitch," that is, the
has a propeller of
propeller is a portion of a screw, the pitch of
which (in one complete turn) would be 37ft.
Length is the portion cut off of the screw, the
useful part; about 1-6th, as above.

66

The "length" is, therefore, measured by a straight line along the "boss," and parallel to its axis. Diameter is the length from the end of one arm to the end of the opposite arm, or the double length of one arm, where the arms are not opposite. The Great Eastern propeller has a diameter of 24ft. Area is the area of the circle described by the points of the diameter. The area of the propeller is therefore that of a circle, the plane of which is at right angles to the shaft of the propeller, and represents the resisting surface of the water. Blades are the arms of the propeller. Here it may be well to note that the area of the blades is not the same as the area of the propeller: in the area of the blades the slanting surface is considered. Slip is the loss caused by the yielding of the water. Theoretically the ship should advance as much as the "pitch" requires-in the Great Eastern, for example, 37ft. Practically the ship does not advance to the full amount of the "pitch;" the water always yields. This loss or slip" varies with the "build" of the vessel, with the shape of the propeller, and from accidental circumstances. Negative Slip is when the vessel is going quicker than the "pitch" of the propeller requires, i..., when the ship advances at each turn over a space greater than the pitch, Suppose the Great Eastern to advance 40ft. at each revolution of the screw, this state of things is called the "negative slip," and, so far from being useful, the propeller is only retarding the progress. When, therefore, negative slip" is discovered, it is time to stop the engines; their action is pure waste. It is evident that some external force-wind, currents, &c.-is able just now to do more for the ship than its propeller. Negative slip is indicated in the "engine The temperature of the globe was then room by an arm against which the shaft of the uniform, the same fossils being found in the Devonian strata wherever it occurs. In the now frigid plains of Russia and the warm and balmy moors of Devon, the same old plants, the same remains of then existing races of animated life appear, and the tree-fern flourished as luxuriantly in the frigid zone as it does now in the torrid regions of the tropics. The chief cause of the convulsions which attended this period was the irruption of Trappean rocks, which appear in intimate association with the Old Red Sandstone formation, in the form of greenstones and felspar porphyries. Little granite is found however, and it appears from this that the granitic period had by this time given way to the Trappean. as the latter has since done in favour of the volcanic. These irruptions considerably vary the aspect of the localities in which the system occurs, but, as a rule, it is somewhat tame, yet here and there forming rounded hills and vales of much quiet beauty. The appearance of the land during the Devonian epoch must have been of a verdant character: covered with forests of the graceful and Though not very specially belonging to the majestic tree-fern and other now tropical plants, propeller, this seems the best place to introduce a whose quiet and silent shades, undisturbed even curiosity which appeared some time ago in the by the hum of insects or the warbling of the Mersey, the "jointed ship." The object of this winged songsters who now people every grove, ship is to avoid the waste and expense consequent

propeller presses when doing useful work. When
this index is not consulted there may be an
apparent negative slip caused by the "pitch" of
the propeller increasing temporarily from the
pressure against the water.

The utility of the propeller is shown by
mentioning its advantages over other means of
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 may
be used only as an auxiliary in calms, &c. When
not acting it can be removed or otherwise pre-
vented from injuring the speed. For merchant-
ships the machinery may be in the "stern,"
leaving ample room for cargo. For war-ships
the propeller is out of sight, can with great diffi-
cnlty 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.

581

this purpose the "connector" is made up of three or more separately complete ships, joined together, and driven by the propeller of the last, Fig. 102.

FIG. 102

When this compound ship has arrived at its dis-
charging wharf, the machinery part is detached
and goes off to seek "another train."
Or it may
happen that the parts of the "connector" have
different destinations to which they are duly con-
veyed 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 of the propeller is quickly told. Now that it has proved such a success there are many It is said that laying claim to the invention. 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 Mr. S. Brown, C.E., London,* who in 1825 had a large ship moved by a screw. There are strong claims made for Mr. John Swan, 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 forward. 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 Admiralty 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 doubt the practical efficiency of the screw as a 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 be found in every class of ship, from the largest man-of-war, oceansteamer, or sailing vessel, to the smallest pleasure yacht.

SCREW 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. The principle on which the new discovery depended was, that to be directed, a body should be heavier than the medium in which it floats. Rejecting the balloon therefore as quite unfit, the inventor proposed to raise the vehicle into the air by means of screw-propellers. scientific toy, four vanes, Fig. 103, spun by a

The

cord (a), or spring (b) rising into 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 yet.

SUB. APP. III.-THE LEECH PROPELLER. Some years ago, under the above title, a pro

peller was invented,

the action of which Was to be that of the leech when swimming. The propeller

suggest to the student musing on the scenes of on the multiplied machinery of several vessels was a flat pointed" band," extending, in a hollow countries a hundred centuries old, a delicious yet lying idle during the discharge of cargo. For awful calm, awakened only by the quiet rustling

of the fern-leaves in the forest or the dull roar of

All rights reserved.

ENGLISH MECHANIC, Vol. IV., p. 220.
Cosmos, Vol. XXIII., p. 141.

It appears from Mr. Grant's valuable paper, read before the Institution of Civil Engineers in December 1865, that Portland cement gains from 20 to 30 per 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 Portland cement as compared with the natural cements; the test blocks were of standard size of 24 square inches, and placed in water as before described:

England from the commencement of the metro- the neck being 14in. by 1 in.; and it must be under-
politan main-drainage works. During the last fif-stood that all experiments referred to have reference
teen years the manufacture of Portland cement has to the weight necessary to sever 21 square inches of
gone on steadily increasing, until at the present neat cement.
day we find that little short of 400,000 tons per
annum are made in the county of Kent-the centre
of cement manufacture irrespective of the pro-
ductions of many minor factories in different parts
of the country.
The chemistry of the setting of Portland cement
is by no means so well understood as it ought to be.
There is no doubt, however, that, like the hydraulic
lime and natural cements, it is, chemically speaking;
a double silicate of lime and alumina; silicic acid
is generated by the hydration of the cement, and
forms insoluble salts with the lime and alumina
bases. It is a curious fact that Portland cement
hardens more rapidly when salt water is employed.
According to Schweitzer, 1,000 grains of sea-water
in the English Channel contain 27-060 grains of
chloride of sodium; soluble silica has a known pre-
ference for alkaline bases, 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, for the sake of distinction, may be termed "Engineers'' cement and "Plasterers' cement. The former is the more costly; it is usually described by manufacturers as "best heavy tested;" it weighs from 112lb. to 120lb. to the bushel, is slow setting, and

Portland cement

Roman cement
Medina cement
Cement de Zumaya
(Spanish)

lb.

Ib.

lb.

119

598

914

1024

76

200

The Builders' Trade Circular vouches for the accuracy of these figures.

same force. The principle, which it is necessary thoroughly to understand, will be best explained by a reference to Fig. 1. Let A C, B C, be two bars 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 equal to 1 ton. By the method of resolution of forces, construct the triangle C D E, and make CD = 1 ton Draw D E parallel to B C, then D E will equal the strain upon B C, and E C that upon A C. If these strains be measured off upon a scale of two tons to the inch, which is the scale upon which C D is drawn, they will be found to equal respectively 0-50 and 0-71. It is not difficult to prove that these values are in the ratio of the cosines of the angles which the bars make with the horizontal line FG. By the principles of trigonometry we obtain in the triangle CDE the proportion D E: EC:: sine ECD: sine ED C. 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 E D C is equal to the angle D C B (Euclid, prop. xv., book 1, which is the complement of the angle B C G, which is equal to 45 degrees. As the sine of any angle is equal to the cosine of its complement, if we call the strain upon the bar BC EDS, and that upon A C EC = S1, we have S: S1:: cosine 60° cosine 45°. Substi

=

tuting the values of these in the equation, we find
05 071 0-50000 0·70710, a sufficiently close
approximation to prove the accuracy of the
diagram.
A similar method is employed in deducing the

of great strength; the latter is a light cement,age; from his experiments it appears that the strains upon the crane in Fig. 2, with the exception

Portland cement is made from chalk and alluvial white chalk, those on the Medway grey chalk; the clay; the factories on the banks of the Thames use latter is probably 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, 6ft. in diameter and 2ft. deep, in which two edge runners," 4ft. 6in. 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,

Mr. Grant's tables show conclusively that the strength of gauged Portland cement increases with breaking weight of rest blocks, one week old, one year old, and two years old, are as 1, 1-5, and 1-62. The ultimate maximum tensile strength has not as yet been ascertained; experiments are, however, being conducted periodically with a view to deter mine this important point. Mr. Grant gives the to the bushel as 7771b., whereas we give it as 10241b., average tensile strength of cement weighing 1191b; 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. the list of our manufactures, but even now its valu. Portland cement now forms an important item in able properties are not as fully appreciated as they deserve to be.

WROUGHT IRON CRANES.-III.

(Concluded from page 587.)

At the same

being thus converted into a fluid state, they filter OUR two preceding articles on this subject crane, such as changing the radii of either the inner

through a band of fine brass-wire gauze fixed to the side of the pan, and flow through wooden "launders" into tanks or settling reservoirs. One wash mill will feed four tanks, each of which is about 100ft. long, 40ft. broad, and 4ft. deep. When one of these has been filled in the manner just described the same process is applied to the others in succession. About three weeks after the tanks are filled the whole of the materials will be precipitated, the clear water being drained off in the mean time through a small weir in the brick side of the tank; the residuum is a plastic mixture of the consistency of "putty," and not much unlike it in colour. The next process is to convey this precipitate from the tank to the "drying floors," over which it is spread in a layer about 6in. thick; each floor is 40ft. by 30ft.; it consists of an outer skin of boiler plates resting on a series of brick ovens and flues. The object of this arrangement is to render the plates sufficiently hot to effect the rapid desiccation of the water from the superincumbent layer, a process generally accomplished in about twelve hours. The materials having thus been thoroughly dried are ready for conveyance to the kilns. The "charge" consists of alternate layers of coke and raw materials, the burning generally occupying thirty-six hours. When the contents of the kiln becomes sufficiently cool, the "clinkers," or cement stones-for the mixture has now assumed that form -are drawn and removed to a floor where the larger pieces are broken, and the whole of the burnt materials are then conveyed to the hoppers of the grinding-mills, where, passing under rapidly revolving horizontal burr-stones, they are ground into an almost impalpable powder. The cement issues from the mill at a temperature of about 160°, and the now manufactured material is wheeled away, and spread in a layer from 2ft. to 3ft. thick over the floor of a cool shed; it is subsequently packed in casks or sacks for conveyance from the works. The essential conditions for the manufacture of good Portland are: 1, The chalk and clay should be thoroughly mixed in the wash-mills, and the fluid materials delivered by launders" over the entire area of the settling tanks. 2, The contents of the kilns ought to be burnt equally throughout. 3, The burnt materials should be ground very fine. 4, After coming from the mill the cement should be spread over the floor of a shed, and allowed to remain there for at least a fortnight previously to being packed into casks or sacks.

that, as we proceed, the resolution of the different such a degree as to hinder any one from mastering forces becomes rather more complicated, but not to the subject with care and attention. time it is not to be supposed that a mere casual thoroughly. Far from it. They must study-not perusal of our article is sufficient to enable our younger readers to comprehend the matter diagrams to a conveniently large scale. For the merely read-it, and work out for themselves the weight of one ton only at the point L, in Fig. 2 sake of simplicity, we will suppose that there is a once the strains due to a weight of one ton have The advantage to be derived from this is that when 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 were confined solely to a practical descrip- or outer flanges, or the angle of inclination of the tion and illustration of the ordinary method of con-bars, will altogether vitiate the calculation. To structing wrought-iron cranes, and we reserved return to our diagram in Fig. 2. Let La equal one for a third and concluding article the question of ton, supported at the front L. How does this protheir theory and scientific analysis. It has been duce strains differing in character and intensity already stated that mathematical formulæ and alge- throughout the whole structure? In the first place, braical equations are not suitable for those particular the weight being supported at the connection of the examples of construction in which the different two bars K L and LH must strain both of them, parts are inclined to one another at angles which compressing the latter and extending the former. not only vary among the parts themselves, but are The parts in compression are shown by thick black also dissimilarly inclined to the horizontal. An lines, those in tension by fine ones. From the example of our meaning is given in Fig. 2, which point a, draw a b parallel to the bar K L, meeting represents a skeleton elevation of a wrought-iron the chord of the part L H of the lower flange in b. Then, measured on the same scale, a b will give the strain upon L K, and Lb that upon L H, and they are respectively equal to 13 and 1.05 tons. The sign minus denotes a tensile strain and + a compressive. Leaving this latter strain alone for a moment, let us trace the further action of that upon LK. This is obviously transferred to point K, where it acts upon G K and K H, extending the former, and compressing the latter. Upon K L plot off Ke equal to a b, equal to the strain upon KL; draw ed parallel to K G, meeting K H in d. The lines c d and K d will give the strains upon G K and K H respectively, and will be found equal to 2-15 and 1.85 tons. Following the strain upon K H, it is transferred to the point H, where it pulls upon the bar G H and compresses HF. 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 Lb, to be taken into consideration, and how it also affects the bars acted upon by the strain upon K 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 K 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 case, and we thus see how important it is to remember these fundamental axioms. The bar G H is acted on by a tensile strain in the direction of K 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.

A

FICI

E

C

bent crane. In practice, the outer and inner flanges
are 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 em-
ploying 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 funda-
mental principle that governs the distribution of
strain upon two separate members inclined at
different angles to the horizontal, and acted upon There are two methods of resolution by which
by the same weight-to speak dynamically, by the these strains on G H and HF 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 KH to e, making He Kd strain upon K H. From e draw ef, parallel to G H. Measure ef and and fH, 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 K H, and will be - 1.0 and making H equal Lb equal the strain upon L H. +24. Now for the other strains, produce LH to l, From 7 draw In parallel to G H, and ln and n H will be the other strains upon G H and H F, and will scale +0.5 and +0.63 tons. Summing up, we obtain the total strains upon these parts of the crane to be as follows:-Upon G H -10+05)

=

==

=

draw

299. 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 ad
mitted to tube tends to straighten it more or less,
according to its intensity. The ends of tube are
the pressure on a dial.
a pinion on the spindle of a pointer which indicates
connected with a toothed sector-piece gearing, with

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.

0.5 tons, and upon HF (+24+0.63) +303 tons. By the second method, the resultant of the strains of KH and LH must be first ascertained. Produce K H to e as before, draw eg parallel to L H, and equal to L b. From g gh, parallel to G H. Then gh will be found, on tube, against which is marked the scale of inches, 301. Mercurial barometer. Longer leg of bent measurement, to equal (e f -ln) 0.5 and h His closed at top, and shorter one is open to the atto equal (Hn+ H)=+3-03 tons. At the point mosphere, or merely covered with some porous G there are also two forces acting-namely, the material. Column of mercury in longer leg, from strain due to the bar G H and that to K G. The which the air has been extracted, is held up by the resulting strains can easily be determined upon the pressure of air on the surface of that in the shorter parts GE and G F by either of the methods we leg, and rises or falls as the pressure of the atmohave just investigated. We cannot spare space to sphere varies. The old-fashioned weather-glass is go through every strain, nor is there any necessity, composed of a similar tube attached to the back of as the determination of the others is simply a repedial, 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.

tition 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.

a

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 revolv. Although this method of calculating strains is ing frame. C is the frame or train-bearing arm. perfectly accurate enough for all practical purposes, 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 is secured a wheel, E, that gears with a wheel, B. may be readily done by very simple formule. For If the first wheel, A, be fixed and a motion be given example, the strain upon the bar KG has been found to the frame, C, the train will revolve around the by the diagram to be equal to 2-15 tons, and it fixed wheel and the relative motion of the frame to may be checked as follows. It is manifest from the fixed wheel will communicate through the train the principle of moments that the strain at any a rotary motion to B on its axis. Or the first wheel point multiplied by the leverage with which it acts, as well as the frame may be made to revolve with will be equal to the weight at that point, multiplied different velocities, with the same result except also by the leverage with which it acts. The strain as to the velocity of rotation of B upon its axis. on the bar G K acts with a maximum leverage of In the epicyclic train as thus described only the

W + L' 17.9

L

=

3.5

gear

304. "Ferguson's mechanical paradox," design to show a curious property of the epicyclic tra The wheel, A, is fixed upon a stationary stud abou In this arm an which the arm, C, D, revolves.

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
wheel, A. three distinct epicyclic trains. Suppose
forming with the intermediate wheel, B, and the
A to have twenty teeth, F twenty, E twenty-one,
and G nineteen; as the arm, E, C, D, is turned
round, F will appear not to turn on its axis, as any
point in its circumference will always point in one
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
the revolution of the arm and pinion will give motion
wheel, C, both concentric with the axis of the arm
Either of the wheels, A, C, may be stationary, and

The wheels, c, d, e and f, constitute an epicycle train of which e is the first and f the last wheel A shaft, A, is employed as a driver, and has firmly cured to it two wheels, a and h, the first of which gears with the wheel, b, and thus communicates motion to the first wheel, c, of the epicyclic train, and the wheel, h, drives the wheel, g, which thus gives motion to the last wheel, f. Motion communicated in this way to the two ends of the train produces an aggregate motion of the arm, 4, 7, and shaft. m, n. This train may be modified; for instance, sup pose the wheels, g and f, to be disunited, g to be fixed to the shaft, m, n, and f only running loose upon it. The driving-shaft, A, will as before communicate motion to the first wheel, c, of the epicyclic train by means of the wheels, a and b, and will also by cause the wheel, g, the shaft, m, n, and the trainbearing arm, k, l, to revolve, and the aggregate rotation will be given to the loose wheel. f

307. Another form of epicyclic train designed for N H, equal to 3.5 feet, whereas the weight at Lacts wheel at one extremity is concentric with the reproducing a very slow motion. m is a fixed shaft with the horizontal leverage of H a, equal to 79 volving frame; but if the wheel, E, instead of feet. If S be the strain required, L its leverage, Wing with B, be made to gear with the wheel, D, upon which is loosely fitted a long sleeve, to the the weight, and L' its leverage in the general for- which like the wheel, A, is concentric with the lower end of which is fixed a wheel, D, and to the frame, we have an epicyclic train of which the upper end a wheel, E. Upon this long sleeve there mula, S = From which we wheels at both extremities are concentric with the is fitted a shorter one which carries at its extremities obtain S = 2.2 tons, which checks the accuracy frame. In this train we may either communicate the wheels, A and H. A wheel, C, gears with both D and A, and a train-bearing arm, m, n, which re of the calculation by diagram. Great care must the driving motion to the arm and one extreme volves freely upon the shaft, m, p, carries upon a always be taken in calculating by diagram, as there wheel, in order to produce an aggregate rotation of stud at n the united wheels, Fand G. If A have 10 is always a tendency for errors to accumulate. One the other extreme wheel, or motion may be given false step at the commencement, if it is not per- to the two extreme wheels, A and D, of the train, teeth, C 100, D 10, E 61, F 49, G 41, and H 51 there ceived, is perpetuated in all the succeeding opera- and the aggregate motion will thus be communicated will be 25,000 revolutions of the train-bearing arm, m, n, for one of the wheel, C. tions, and would ultimately cause a very serious to the arm. difference in the correct values. But by using the 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 News.

303. A very simple form 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 order to produce aggregate motion of the other wheel.