SUMMARY OP HYDRAULICS. £3 



- To Ihow the advantage of breaft wheels over common un- Breaft wheel* 



derfbot wheels, the author quotes Mr. Banks's experiments. ™ uch be " e 5 

 TT , , .*. i i r, i .1 than undertook 



He alio obterves, that by placing two wheels after each other 



in the fame ftream, about one fourth more force may be ob* 



tained than either by a tingle wheel, or by two wheels fide by 



fide; but that a tingle wheel has lefs friclion, and is generally 



lefs expenfive. 



Chapter 14. Of the properties of the air, as far as they relate 

 to hydraulic machines. 



What Mr. Eytelwein quotes as Mariotte's difcovery of the Force of air 

 increafe of the air's deniity in proportion to the preflure, was a S a5nft a plane i 

 well known to Hooke and Boyie. From the experiments of 

 Woltmannand Schober, he remarks, that the force of the wind 

 againft a perpendicular plane, is nearly equal to four thirds of 

 the weight of a column of air, of a length equal to the height 

 due to the velocity. The height of a column of water nearly 

 equivalent to the force or refinance may be found, by taking Rule for finding* 

 the fquare of —^ of the velocity in a fecond, in Englifli feet. lC * 



Thus, if the velocity were 1000 feet in a fecond, the refif- Example, 

 tance would be equal to a column of water in the fame fur- 

 face, 25 feet in height ; and the refiftance to a fphere about 

 half as much. 



For another example, if we had a cubic foot of a fubftance Inftance of the 

 equal in fpecific gravity to water, and were deflrous of know- utrr j oft velocity 

 • .i x o i •/ .i , •, .i • i r „• . , poffible to be had 



ing the greatelt velocity that it could acquire by railing through by a body falling 



the air; the height of the column of water is here 1, and its in the air: 

 "fquare root I , which multiplied by 200 gives 200 feet in a fecond 

 for the velocity, when the refiftance would be equal to the weight, 

 which of courfe is the limit beyond which the velocity could 

 never pafs. Hence we may form an idea of the u(moft velocity 

 that a ftone, of moderate fize, could acquire in defcending 

 from the upper regions of the atmofphere, or even from the 

 neighbourhood of the moon ; a velocity that would be much much lefs than 

 lefs than that of a bomb or a cannon ball, even when it may be ball, 

 followed by the eye. 



Again, Mr. Garnering parachute contains about 860 fquare Parachute of 

 feet of furface, and weighs, together with the aeronaut f u f, Garnerm » 

 pended from it, about 230 pounds. Here the weight is f~| of 

 a pound for, each fquare foot, which is equivalent to ^i-o °f a 

 foot of water; multiplying the fquare root by 200, we have 

 about 1 3 feet in a fecond for the utmoft velocity ; which is the 



G2 temjp 



