ON THE REGULATION OF HYDRAULIC FORCES. 243 



sures, or by instruments founded on the laws of hydraulic pressure. One 

 of the best of such instruments is the tube invented by Pitot,* and improved 

 by Buat.t A funnel is presented to the stream, and the water in a vertical 

 tube connected with it is elevated above the level of the river, nearly to the 

 height corresponding to the velocity : but it is said that the result will be 

 less liable to error, if the funnel be covered by a plate with a small orifice in 

 its centre, the elevation being in this case always half as great again as the 

 height due to the velocity. Other instruments intended for the | same pur- 

 pose, require some previous experiments for determining the degree in 

 which they are affected by different velocities ; in this manner the hydro- 

 metrical fly is adjusted ; the impulse of the water on two inclined planes 

 turning an axis to which they are fixed, and by its means a series of wheels, 

 with an index, which expresses the space described during the time of 

 observation. Instruments similar to these have also sometimes been em- 

 ployed, for measuring the relative velocity with which a ship under way 

 passes through the water ; and an apparatus, resembling Pitot' s, has been 

 adapted to this purpose by Captain Hamilton, with the addition of a tube 

 inserted into it on a level with the surface of the water, which continually 

 discharges a small stream into a reservoir with a velocity regulated by the 

 pressure, and consequently equal or proportional to that of the ship itself. 

 In this manner he obtains an accurate register of the whole distance 

 described, including the effect of all the variations of the velocity. If the 

 orifice be small, it will be necessary to attend to the temperature of the 

 water, since the discharge is considerably retarded by any considerable 

 degree of cold. But when the aperture which determines the magnitude 

 of the discharge is wholly under water, as Captain Hamilton has placed 

 it, this source of error is probably much diminished. (Plate XXII. Fig. 

 288, 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 are 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 



* Hist, et Mem. del'Acad. de Par. 1732, p. 263, H. 103. 

 f Principes d'Hydraulique, vol. ii. See also 



Langsdorffs Hydraulik, PI. 25. 



Brouckner's Machine, Hist, et Mem. de Paris, 1750, H. 169. Woltmann, 

 Theorie des Hydrometrischen Fliigels, Hamb. 1790. 

 " 



. 



"Papers on Naval Architecture, Repert. ii. I. 355. 

 || Such as Lind's Wind-Gage, Ph. Tr. 1775, p. 353. 



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