DESCRIPTION OF PLATES. xxiii 



PLATE XXI. 



Fig. 268. A stream of air being forced through the pipes A and B, the mercury 

 in the barometer C D falls from C to D. P. 225. 



Fig. 269. A stream entering the reservoir A, by the pipe B, carries with it all 

 the water C, which stands above the level of its upper surface. P. 226. 



Fig. 270. The ball A is permanently supported by the jet B, because when it 

 falls into the position here represented, the centrifugal force of the water at A carries 

 it back to the middle of the jet. P. 226. 



Fig. 271. A plate, bent into the form ABC, turning on the centre B, is 

 impelled by a stream of air D in the direction C D. P. 226. 



Fig. 272. A cylinder moveable on an axis, with two curved pipes inserted in its 

 lower part, seen from above. The stream A enters at the top of the cylinder, and is 

 discharged by the orifices B, C, so as to turn the vessel in the direction B D. 

 P. 229. 



Fig. 273. A jet of a fluid, striking on an obstacle of equal diameter, and separated 

 by it so as to continue its motion obliquely. P. 229. 



Fig. 274. The whole resistance directly opposed to the surface A B being repre- 

 sented by B C, the portion which, according to the principles of the resolution of 

 forces, ought to act on the wedge A B D, is represented by B E ; and in the same 

 manner the resistance on A B F is to the whole as B G to B C. P. 230. 



Fig. 275. The form of the dead water moving before an obtuse body is nearly 

 like that of A B C ; and the form adapted for moving through the water with the 

 least possible resistance like A B D C. P. 231. 



Fig. 276. The direction in which the particles of a fluid are supposed to move 

 when they strike against a concave surface. P. 232. 



Fig. 277. A hydrostatic balance. P. 235. 



Fig. 278. Mr. Nicholson's hydrometer, to be employed with weights, for finding 

 the specific gravity of fluids or solids. P. 236. 



Fig. 279. A spirit level. P. 237. 



Fig. 280. An overflowing lamp. The hemispherical counterpoise, which is so 

 loaded, that its centre of gravity is at A, raises the surface of the heavy fluid B the 

 higher as it is more exhausted, so that the oil C is always forced up nearly to the 

 level of the wick at D. The oil is poured in by a pipe, in the middle of the 

 cylindrical column. The air holes may be made wherever it is most convenient. 

 P. 237. 



Fig. 281. A section of an embankment, of a proper form to be opposed to the sea, 

 with a drain passing through it, and a valve at its opening. P. 238. 



Fig. 282. The form recommended for the section of a river or canal. P. 238. 



Fig. 283. A B shows the strongest form for a vertical beam, fixed above and 

 below, and calculated to resist the pressure of a fluid ; the greatest thickness being 

 at C ; and D E is the outline of a series of horizontal planks, of such a thickness as 

 to afford equal strength throughout the sluice or floodgate. P. 239. 



Fig. 284. A box, with a valve supported by a hollow ball, for letting out air from 

 pipes, when it is below the level of the reservoir. P. 241. 



Fig. 285. Two methods of letting out air from pipes, when it is above the level of 

 the reservoir ; A a valve with a stopcock near it ; B a vessel of water, screwed on for 

 receiving the air; to be replenished with water as it becomes empty. P. 242. 



Fig. 286. A section of a compound stopcock, which receives a fluid from either 

 of the pipes A, B, or C, into a cavity which descends a little in the direction of the 

 axis, and communicates with the pipe D, by means of one of the bores represented 

 by dotted lines, according to the position into which the moveable cylinder is turned. 

 P. 242. 



Fig. 287. Valves of different kinds ; A the common clack valve ; B a double 

 clack valve, consisting of two semicircular valves ; C a pyramidical valve, consisting 

 of four triangular pieces ; D a circular valve turning on an axis ; E a steam valve 

 of metal, sometimes called a T valve ; F, a valve of oiled silk or bladder, supported 

 by a grating, for air. P. 242. 





