ox THE ItlGUtATION OF HYDKAULIC rORCES. 319 



water, since the discharge is considerably retarded by any considerable degree of 

 cold. But when the aperture, which determines the magnitude of the discharge, 

 ^s .wholly under water, as Captain Hamilton has placed it, this source of error 

 is probably much diminished. (Plate XXII. Fig. 1288, 289.) 



The motions of the air may also be measured by instruments similar to 

 those which are employed for determining the velocity of streams of water. 

 The direction of the wind is sometimes indicated by a wind dial, consisting 

 simply of an index, connected by wheels with a common vane or weather- 

 cock. Its velocity may be found by means of wind gages of different kinds : 

 these arc sometimes constructed by opposing a flat surface to the wind, the 

 pressure being measured by the flexure of a spring, or by the winding up of 

 a weight on a spiral barrel ; and sometimes by receiving the stream in the 

 mouth of a funnel, so as to raise a column of water, in a vertical tube, to a 

 height equivalent to the pressure, or to condense a quantity of air inclosed in 

 a cavity, to a degree which is indicated by the place of a small portion of 

 mercury, moving in a horizontal tube, which leads to the cavity. A little 

 windmill, like the hydrometrical fly, may also be employed for measuring the 

 velocity of the wind, with the assistance of a watch. 



The principal methods of applying the force of fluids to useful purposes are 

 to employ their weight, their impulse, or their pressure. The weight of 

 "w^ater may be applied, by collecting it in a reservoir, which alternately ascends 

 and descends, by causing it to act within a pipe on a moveable piston, or by 

 conducting it into the buckets of a revolving M-hecl ; its impulse may be di- 

 rected either perpendicularly or obliquely against a moveable surface ; and its 

 pressure may be obtained, without any immediate impulse, by causing a 

 stream to flow horizontally out of a moveable pipe which revolves round an 

 axis. The force of the air can only be applied by means of its impulse, and 

 this may be employed either perpendicularly or obliquely. 



When water is collected in a single reservoir, which serves to work a pump 

 or to raise a weight, the mode of its operation may be determined from me- 

 chanical considerations only ; and it is obvious that if we are desirous of pre- 

 serving the whole force of the water, we must employ a second reservoir to 

 be filled during the descent of the first, which may either descend in its turn, 



