7C8 



MECHANICAL PHILOSOPHY. PNEUMATICS. [ATMOSPHERIC PBRSSCKH. 



Fig. 1 gives an nnd view of the machine. A A are 



it support*, generally aboat eighteen feot higli. 



B is tho base on which they are fixed. is a fly-wheel 



for . [ii.i'isni.,' the motion; and kk are winches turn- 

 Under .Hi win. -a a rojw is wound. By means 

 of the rope, the "monkey," K, a heavy man of metal 

 attached to it by the head F at /, U raited by a pulley to 

 the top of the upright* at m. The mode of detaching 

 the monkey, after it* being railed to the top, is thus 

 managed : In < (Fig. 1) we notice an arrangement like 

 a pair of pincers. The same may be seen in Fig. 3, in 

 which K, a crou-bar, receives the axis of these clamps. 

 I u l-'i.' 4, n represents two rings, from each of which 

 a cord U hung, so that they may be hold in tho hands 

 of the workmen beneath. Now, supposing the "monkey" 

 to have been raised to the top of the machine, and th.-it 

 ita lower part is vertically over a pile, the men in attend- 

 ance pull the cords stretched from n n (Fig. 4), and the 

 damps at once open and let fall the monkey. Tho 

 separated clamp is represented in Fig. 5, witli the conl as 

 stretched from the hand of the person holding it. Tho 

 "monki-y," lalliii'.; a height of some sixteen or twenty 

 feet, acquires a velocity of nearly forty feot per second ;* 

 and being of great weight, its momentum f is sufficient 

 to drive the pile receiving it with great force into the 

 earth. The "monkey" is then again attached to the 

 clamps, e, and raised to the top of the machine, again 

 to fall. The operation is repeated until the pile has 



been driven sufficiently into tho soil. Fig. 2 is a side view 



i>f tho same machine, with tho exception that I 



ui'Mit of toothed wheels is employ,. 1, as shown a' 



is a block tix.'d to tlm uprights A. for tin s.v 



giving it additional support Som 



extended, as at o a, and these joists are bolted firmly 



into a platform on which the machine stands. 



Fig. 6 illustrates an application of hydrostatic laws. 

 The principle of tho hydrostatic power has already bean 

 explained at page 750. In this case, it is applied for tha 

 purpose of removing piles, which have been driven into 

 the earth by the pile-driving machine. A is a bed of 

 piles requiring to be removed, B being one of thorn. 

 C D is a block of timber, which acts as a lever, whose 

 fulcrum is E. This lever belongs to the second order,! 

 the resistant or weight being in the lines I, m, n. Tlio 

 latter letter shows the chain by moans of which tho pile 

 is attached to the lever : d is the head of the piston, and 

 moves through a collar 6. The piston is inside tho 

 cylinder F. The pump which works the press may now 

 bo described : h is its handle, working on a pivot </ ; f 

 is the plunger, which works in the cylinder k. Water is 

 thus driven through tho pipe e into F ; and the power 

 thus produced effects the removal of the sunken pile B 

 by tho action of the lever, itc. It will be seen that the 

 principles of the lever, and tho pressure of liquids, are 

 both employed in this ingenious arrangement. A similar 

 arrangement is adapted to hoists, cranos, <tc. ED.] 



CHAPTER VI. 

 PNEUMATICS. 



THE fluids treated of in the foregoing chapter are all re- 

 garded as inelastic or incompressible : it remains for us 

 to devote a few pages to the consideration of those of an 

 elastic or compressible nature. The most important of 

 these is the atmosphere with which we are surrounded : 

 and it is in especial reference to this, that the examination 

 of the mechanical properties of elastic fluids is said to 

 belong to the science of Pneumatics derived from a 

 Greek term signifying, the air we breathe. 



TRANSMISSION OF PRESSURE : COMPRES- 

 SION. The mechanical properties of elastic and in- 

 elastic fluids are in many important particulars the same: 

 the fundamental principle in Hydrostatics, for instance 

 namely, that a pressure applied to the surface of a 

 fluid, is transmitted undiminished in all directions 

 throughout the entire volume of the fluid holds equally 

 whether the fluid be elastic or inelastic ; and in both 

 cases may be put to the test of experiment in the same 

 way. If the piston A, shown at page 748, play in a 

 metal cylinder projecting into the interior of the vessel, 

 and be forced inwards along the tube by a pressure or 

 weight of 1 Ib. say, it will advance under this pressure 

 along the tube, and then stop : and it will be found that 

 an additional pressure of 1 Ib. must be applied to the 

 piston B to prevent its being forced out. 



I n the case of a liquid, the only effect of the pressure 

 on A is the transmission of it to every portion of the 

 surface of equal area. In the case of the air, there is 

 another and a distinct effect observably produced 

 namely, the compression of the fluid into smaller volume; 

 and it is found by experiment that the volume diminishes 

 just as the pressure increases : in other words, that the 

 pace into which any quantity of air is forced to compress 

 iUelf, U inversely as the pressure applied. How this 

 truth was ascertained we shall explain presently : it is 

 necessary first to establish tho fact that air has weight. 

 It U possible to conceive that a substance like air may 

 have the property of transmitting pressure, yet that it 

 may not lUeU be a prowing or weighty substance. 



\l, PROOF OF ATMOSPHERIC 

 l'l:i:>^Il:K - r,ui that the air exercises pressure, may 

 be proved by many easy experiments : the boys' sn< 

 IN <,. rat. tBMMfep.ru. 



is familiar to everybody. This toy consists of a piece of 

 leather with a string passing through a perforation in 

 the middle : npon being wetted so as to exclude the air, 

 when it is pressed close to a smooth surface, it is found 

 that considerable force must be applied to the string to 

 detach tho sucker ; and if the surface of it be large, a 

 stone of considerable weight may be thus lifted. What 

 is it that thus presses the sucker and stone so firmly 

 together ? We are compelled to answer, the air, which 

 surrounds them both as if they were one body. Before 

 the sucker was applied, the air surrounded it : the pres- 

 sure, if any, was alike on its upper and under surface : 

 but when affixed to the stone, the pressure on the under 

 surface of it was excluded, while that on the upper sur- 

 face remained the same ; and as an equal pressure is 

 exerted on the corresponding area of tho under surface 

 of the stone, the two are, as it were, thus pinched or 

 1 together. 



Again : take a glass tube, a foot or two long, open 

 at both ends, and therefore full of air ; plunge one end 

 into a vessel of water : then we know from the first 

 principles of Hydrostatics, that the level of the water 

 will be the same both outside and within the tube. Let, 

 now, the air in the tube be drawn out by the mouth, or 

 pumped out by a machine fitted for the operation : tho 

 water will be seen to rise within, and at length to fill the 

 tube ; and if the thumb be quickly applied to the top, 

 so as to prevent the readmissipn of the air, the v 

 thus filling the tube will remain suspended, just as it 

 would do if an additional column of water of the same 

 height as the unimmersed part of the tube pressed upon 

 the surface of the water in the vessel. In the absence 

 of this additional column of water, what is it that sus- 

 tains the water in the tube ? We are again compelled 

 to answer, tho pressure of tho air which supplies its 

 place. 



We have hero supposed the tube to bo only a foot or 

 two long, for convenience ; but careful experiment has 

 proved, that with a tul>e of sufficient extent, exhausted 

 of air, the water would not cease to rise till it had at- 

 taiiu-d the elevation of about 32 feet ; which limit at- 

 tained, it would remain stationary. On this large scale 

 t Bctanti, p. 719. 



