On the Rcfijlance of Bodies moving in Fluids. 



5" 



the apparatus being thus adjufted, open the (lop cock, and let the fluid ftrike the lever, and 

 put fuch weight into the fcale as will firft keep the lever in its perpendicular fituation, and 

 that weight with the weight of the fcale muft bejuft equivalent to th»a£lion of the fluid. 

 Thus we get the perpendicular efl'e£l of the water. Now incline the plane of the lever at 

 any angle to the direction of the ftream, and adjuft the firing perpendicular to the plane as 

 before ; then put fuch a weight into the fcale as will keep the lever perpendicular to the 

 horizon whilft the fluid a£ls upon It, and you get that part of the efl^e£l of the fluid which 

 a£ls perpendicular to the plane. In this manner, when the fluid a£ls oblique to the plane, 

 we get the perpendicular part of the force. The fecond column of the following table 

 (hews this efFe£l by experiment, for every tenth degree of inclination (liewn in the firft 

 column, and the third column fliews the effeCt by theory from the perpendicular force, 

 fuppofuig it to vary as the fine of inclination. 



It appears from hence that the re(ifl:ance varies as the fine of the angle at which the fluid 

 ftrikes the plane ; the difference between the theory and experiment being only fuch as 

 may be fuppofed to arife from the want of accuracy, to which the experiments muft 

 nece(l^rily be fubje£l. 



Let us now firft confider what the whole perpendicular refiftance by experiment is when 

 compared with that by theory. Now by theory the refiftance is equal to the weight of a 

 column of the fluid, whofe bafe =0,045 i"c. and altitude =45)1 Inc. and the weight of that 

 column Is = i oz. i dwt.. lO grs. Hence refiftance by theory : the refiftance by experi- 

 ment : : I oz. 1 dwt. 10 grs. : 1 oz. 17 dwts. 12 grs. : : 514 : 900. 



In the next place let us examine what is this refiftance compared with the refiftance of 

 a plane moving in a fluid. We here prove, that the refiftance of the fluid in motion a£ling 

 on the plane at reft : the refiftance by theory : : 900 : 5 14 ; and we have before proved that 

 the refiftance by theory : the refiftance of a plane body moving in a fluid : : 1598: 2321 j 

 hence the refiftance of a fluid in motion upon a plane at reft, the refiftance of the fame plane 

 moving with the fame velocity in a fluid at reft : ; 900 x 1598: 514x2321 :: 143 8200 : 1192954 

 : -.6 : 5 nearly. Now we know that the a£lual effcd on the plane muft be the fame in both 



3 U 2 . cafes; 



