512 On the Refijlance of Bodies moving in Fluids. 



cafes ; and the difference, I conceive, can arife only from the a£Hon of the fluid behind the 

 body In the latter cafe, there being no efFe£t of this kind in the former cafe. For in re- 

 fpea to the prefluri before the body, that will probably be the fame in both cafes ; for there 

 is a preffiire of the column of the fpouting fluid adting againfl: the particles which ftrike the 

 body at reft, fimilar to the a£lion of the fluid before the body upon the particles which ftrike 

 the body moving in the fluid. Hence the refiftance of the planes moving in the fluid with 

 the velocity here given, is diminifhed about one-fifth part of the whole by the preflurc 

 behind the body j but with different velocities this diminution muft increafe as the velocity 

 increafes. 



The effect of that part of the force which aded perpendicular to the plane being thus efta. 

 blifhed, we proceed next to examine what part of the whole force which afts parallel to the 

 plane is effeclive. To determine which, the axis w v (Fig. 2.) was fixed perpendicular to the 

 plane of the lever abed, and the ends of the axis were conical, and laid in conical holes ; 

 and the thread from which the fcale was hung was fixed to the edge at e, and a£l:ed perpen- 

 dicular to it, and the weight drew the lever in thedire£tion#j-, contrary to that in which the 

 fluid tends to move the lever, and it atled at the fame perpendicular dlftance from the axis 

 below as the fluid a£l:ed above it. Let ;v w z be a line parallel to the horizon when the lever 

 is perpendicular to it, and which pafles through the centre of the ftream, and let .v »» z be 

 alfo,the diredtion of that part of the force which afts parallel to the plane. This apparatus 

 Ipeing adjufted, the experiments were made for every tenth degree of inclination ; and here 

 a circumftance took place for which I can give no fatisfaftory reafon. Having gone through 

 the experiments once, and noted the refults, I repeated them; and to my great furprize I 

 found all the fecond refults to be very different from the firft. The experiments were there- 

 fore repeated again, and the refults were ftill difl"erent. Being certain that the experiments 

 were very accurately made each time, I was totally at a lofs to conje£lure to what circum- 

 ftance this difference of refults was owing. By repeating, however, the experiments, and 

 obferving at what point of the line x m z, the centre of the ftream, a£led, 1 difcovered that 

 the effedt varied by varying that point, that it was greateft when the ftream ftruck the lever 

 as near as it could to x ; lefs when it ftruck it at the middle w, and leaft when it ftruck it as 

 near as it 'could to z, notwithftanding that the ftream a£ted at the fame perpendicular dif- 

 tance from the axis in each cafe, and the parallel part of the force always afted in the line 

 X mz. At the angles 8o° 70° 60° the fluid ftriking as near as it could to the edge z, gave 

 the lever a motion not in the direction x m z^ but in the oppofite diredlion z tn x,zs appeared 



by taking away the fcale. I have therefore marked fuch refults with the fign , the 



motion produced being then in a dire£tion oppofite to that which ought to have been pro- 

 duced by that part of the force of the ftream which a£i:s parallel to the plane of the lever; 

 The forces which are here put down are thofe which take effe£l in a dire£tion parallel 

 to the plane of the lever for every tenth degree of inclination ; the perpendicular force being 

 loz. i7dwts. I2grs. 



At 



