318. LECTURE xxvrr. 



this condition better than the common clack valve of leather, which is generally 

 either single, or divided into two parts; but it is sometimes composed of four 

 parts, united so as to forma pyramid, nearly resembling the double and triple 

 valves which are formed by nature in the hearts of animals. A board, or a 

 round flat piece of metal, divided unequally by an axis on which it moves, 

 makes also a very good simple valve. Where a valve is intended to intercept 

 the passage of steam, it must be of metal ; such a valve is generally a flat 

 plate, with its edge ground a little conically, and guided in its motion by a 

 wire or pin. For air, valves are commonly made of oiled silk, supported by a 

 perforated plate or grating. (Plate XXI. Fig. 286, 287.) 



Before we consider the application of the force of fluids in motion to prac- 

 tical purposes, we must attend to the methods of measuring the velocity of 

 their motions. This may be done either by a comparison with linear mea- 

 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. 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 purpose, require some 

 previous experiments for determining the degree in which they are' affected by 

 different velocities ; in this manner the hydrometrical 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. Instamients similar to 

 these have also sometimes been employed, for measuring the relative velocity, 

 with which a ship under way passes through the water ; and an apparatus, re- 

 sembling 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 M'hole 

 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 



