that is to say, the pitch of the screw on C D varies with the number of teeth on its wheel a.

Let k and k, be the number of threads per inch on the cylinders C D and F E respectively, then

Now, let there be an intermediate pinion and wheel, turning on the same axis, placed between A and B; and let the pinion (acted upon by A) contain e, teeth, and the wheel e teeth; then the velocity ratio of the axis F E will be increased by the ratio e and hence eq. (3) becomes

e

To produce a changing reciprocating rectilinear motion by a combination of the camb and screw.

110. E F is a conical shaped camb, turning on the eccentric axis A B, on which is cut the screw K B, working in the fixed nut

E

Fig. 81.

"

F

B

or hollow screw N; D C a rod resting on the camb, constrained to move in the direction of its length, and to which the varying reciprocating motion is to be given. Here, whilst the camb revolves, it has a continuous motion in the direction of the axis A B, so that the lower extremity c of the rod D C describes a

spiral or screw curve upon the cone whose pitch is equal to the itch of the screw K B. The effect of this is to make C D reciprocate in its path in such a manner that the stroke in one direction is shorter than that in the opposite direction.

To produce a boring motion by a combination of the screw and toothed wheels.

111. Here it is required to produce a rapid rotation combined with a very slow motion in the direction of the axis.

The screw I B is cut upon a portion of the revolving axis A B; this screw passes through a

nut K capable of revolving

with the wheel G, but in

capable of moving in the direction of its axis, as in Case 2, page 59; the wheel & is driven by the pinion F revolving on the parallel axis D C; E is

Fig. 82.

a long pinion, turning on this axis, and acting on the wheel L, which transmits a rotatory motion to the screw axis A B. Now, the rotation of CD produces a rotatory motion in the axis A B, and at the same time causes it to advance, in the direction of its length, with a velocity determined by the following formula.

Let Q, Q1, 91, be the synchronal rotations of the axis c D, the nut K, and wheel &, and the wheel and axis A B, respectively; N, N1, n, n1, the number of teeth in the wheels F, G, E, L, respectively; s the space moved over by A B in the direction of its length; and t = the pitch of the screw I B.

Now, Q, rotations of the nut K move the screw a B through a space equal to Q1 x t; but q, rotations of L move the screw through a space, in the opposite direction, equal to q, x t; therefore in Q rotations of the axis C D the screw A B will be moved through a space equal to the difference between qxt and qxt; that is,

S=

but 21
Q1

Now, the difference

n

N

= and
N

n

ni

N

n1 N

N

n = ; Q n1

Qt... (1).

may be very small as compared

with Q, and consequently s may be made as small as we please

as compared with Q, which is the condition required for the construction of a boring instrument. The boring tool is placed upon one extremity of the axis A B.

66

SECTION III.

ON PRIME-MOVERS.

CHAPTER I.

ON THE ACCUMULATION OF WATER AS A SOURCE OF MOTIVE POWER.

THE MACHINERY of mills, as a whole, may be generally divided into three classes :-the prime-movers, from which the power is derived for keeping the machinery of the mill in motion; the transmissive machinery or millwork (shafting, gearing, &c.), by which the power obtained through the prime-mover is distributed over the different parts of the mill, so that it may be applied at the most convenient place and at the required velocity; and lastly, the machines, technically so called, by which the special operations of the mill in the preparation of its manufactures are carried out. It will be convenient to treat of these divisions in separate sections and in the order just named.

Prime-movers are those combinations of mechanism which receive motion and force directly from some natural source of power, and convert it into that condition in which it is applicable to the purposes of manufacture. Thus the water-wheel takes from the falling water a part of the work accumulated in it, and imparts it as a rotatory motion to the machinery of the mill; and, similarly in the steam-engine, the heat force of the fuel is converted through the medium of the pressure of the steam into motive power in a condition for producing work or mechanical effect. Also the force of currents in the atmosphere impinging upon the expanded sails of windmills has been in former days extensively employed as a motive power. From these three sources, falling or moving water, the combustion

of coal in the production of steam, and wind, we derive almost exclusively at the present time the motive power necessary for carrying on our immense mining and manufacturing systems.

It is only of late years that in this country the steam-engine has nearly superseded the use of air and water as a prime-mover. Until recently steam has been auxiliary to water; it is now the principal source of power, and waterfalls are of comparatively small value, except in certain districts. So long as water was depended upon, the mills of Great Britain and Ireland were necessarily circumscribed in their operations and diminutive in size; they have now become so colossal that they require steamengines of much greater power than the largest water-wheels, and there appears to exist no limit to the magnitude and importance to which they may yet attain.

Water-wheels, therefore, are those prime-movers which receive a certain portion of their energy from falling or flowing water, and their power or dynamic effect clearly depends upon the amount of water supplied and the height through which it falls, or its velocity at the point of application. Hence waterwheels are usually placed on the banks of rivers where à large body of water is at hand, and near some considerable natural or artificial fall in the bed of the stream.

A curious and interesting phenomenon occurs in the neighbourhood of Argostoli, and is taken advantage of, as described in Ansted's Ionian Islands, in the following manner:

At four points on the coast, the sea, at its ordinary level, enters a very narrow creek, or broken rocky channel, and after running somewhat rapidly through this channel and among broken fragments of rock for a short distance, it gradually becomes sucked into the earth and disappears. By conducting the water through an artificial canal for a few yards, and so regulating its course and forcing all the water that enters to pass in a single stream beneath an undershot wheel, power enough is obtained in two cases to drive a mill. Mills have, in fact, been placed there by an enterprising Englishman, and are constantly at work. The stream, after being utilised, is allowed to take to its natural channel, and is lost among the rocks.

'It is common enough to drive a wheel by a current of water going from the land towards the sea; but it is certainly rare, and, as far as I am aware, peculiar to the locality, to find mills driven by a current of sea-water acting quite independently of tide, the water constantly and steadily rushing in over the earth's surface and finally disappearing.

'The general condition of the surface is as follows:

'The small harbour of Argostoli is enclosed on both sides by the hard, broken, limestone rock so common in the islands. On the east side it rises immediately into hills of moderate elevation; and on the west side, behind the town, there is a plateau, scarcely above the usual level of the water, rising about two or three hundred yards from the shore into a low ridge, which, in fact, by its projection into the gulf, makes the harbour. Between the shore line and this low ridge there is an evident depression of the surface in all that part over which the sea when it enters is sucked in. There is evidently beneath this part an extensive cavernous tract, which may well hold much more water than during any ordinary season or succession of seasons can drain naturally into it, in consequence of the rainfall at the surface.

'But what, it will be asked, becomes of the waters of the sea thus pouring in continually to fill the cavern? Certainly, in time, any cavity must be filled, if it has no natural outlet, and if water is constantly entering it. How, also, can the water run off, if its level in the cavern is below the sea-level? It is not, perhaps, so difficult as may be thought to answer these queries.

The water that everywhere enters the earth is always circulating. It not only pours down into and amongst all rocks, but is afterwards lifted, and the level of these subterranean stores is greatly elevated by operations going on at the surface, often at a great distance above.

The cause of this is evaporation, which proceeds incessantly from the surface of all rocks, but especially from limestones. The narrow crevices, common in limestone rocks, act as capillary tubes. When water falls on the surface of such rock, it finds its way down readily, and this seems quite natural; but