ON THE REGULATION OF 1IY13KAUUC FORCES.

31.9

water, since the discharge is considerably retarded by any considerable degree ofcold. But when the aperture, which determines the magnitude of the discharge,is wliolly under water, as Captain Hamilton has placed it, this source of erroris probably much diminished. (Plate XXII. Fig. 288, 289.)

The motions of the air may also be measured by instruments similar tothose which are employed for determining the velocity of streams of water.The direction of the wind is sometimes indicated by a wind dial, consistingsimply 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 are sometimes constructed by opposing a Hat surface to the wind, thepressure being measured by the flexure of a spring, or by the winding up ofa weight on a spiral barrel; and sometimes by receiving the stream in themouth of a funnel, so as to raise a column of water, in a vertical tube, to aheight equivalent to the pressure, or to condense a quantity of air inclosed ina cavity, to a degree which is indicated by the place of a small portion ofmercury, moving in a horizontal tube, which leads to the cavity. A littlewindmill, like the hydrometrical fly, may also be employed for measuring thevelocity of the wind, with the assistance of a watch.

The principal methods of applying the force of fluids to useful purposes areto employ their weight, their impulse, or their pressure. The weight, ofwater may be applied,by collecting it in a reservoir, which alternately ascendsand descends, by causing it to act within a pipe on a moveable piston, or byconducting it into the buckets of a,revolving wheel; its impulse may be di-rected either perpendicularly or obliquely against a moveable surface; and itspressure may be obtained, without any immediate impulse, by causing astream to flow horizontally out of a moveable pipe which revolves round anaxis. The force of the air can only be applied by means of its impulse, andthis may be employed either perpendicularly or obliquely.

When water is collected in a single reservoir, which serves to work a pumpor 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 tobe lilled during the descent of the first, which may either descend in its turn