VOL. LXXXVIII.] 



PHILOSOPHICAL TRANSACTIONS. 



253 



moving in the fluid. Hence, the resistance of the planes moving in the fluid, with 

 the velocity here given, is diminished about -f part of the whole, by the pressure 

 behind the body; but, with different velocities, this diminution must increase as the 

 velocity increases. 



The effect of that part of the force which acts perpendicular to the plane being 

 thus established, we proceed next to examine, what part of the whole force which 

 acts parallel to the plane, is effective. To determine which, the axis wv, 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 scale was 

 hung was fixed to the end at e, and acted perpendicular to it, and the weight drew 

 the lever in the direction es, contrary to that in which the fluid tends to move the 

 lever, and it acted at the same perpendicular distance from the axis below, as the 

 fluid acted above it. Let xmz be a line parallel to the horizon, when the lever is 

 perpendicular to it, and which passes through the centre of the stream ; and let 

 xmz be also the direction of that part of the force which acts parallel to the plane. 

 This apparatus being adjusted, the experiments were made for every 10th degree of 

 inclination ; and here a circumstance took place, for which I can give no satisfac- 

 tory reason. Having gone through the experiments once, and noted the results, I 

 repeated them ; and, to my great surprise, I found all the 2d results to be very dif- 

 ferent from the first. The experiments were therefore repeated again, and the 

 results were still different. Being certain that the experiments were very accurately 

 made each time, I was totally at a loss to conjecture to what circumstance this dif- 

 ference of results was owing. By repeating however the experiments, and observing 



at what point of the line xmz the centre of the stream acted, I discovered that the 



effect varied by varying that point; that it was greatest when the stream struck the 



lever as near as it could to x\ less when it struck it at the middle m; and least 



when it struck it as near as it could to z, though the Inclin. _ dwt. gr. 



stream acted at the same perpendicular distance from the 80 c 



axis in each case, and the parallel part of the force always 



acted in the line xmz. At the angles 80°, 70°, 60°, the 70 



fluid striking as near as it could to the edge z, gave the 



lever a motion, not in the direction xmz, but in the op- g 



posite direction zmx, as appeared by taking away the scale. 



I have therefore marked such results with the sign , 



the motion produced being then in a direction opposite to 



that which ought to have been produced, by that part of 



the force of the stream which acts parallel to the plane of 



the lever. The forces which are here set down, are those 



which take effect in a direction parallel to the plane of 



the lever, for every 10th degree of inclination; the per- 

 pendicular force being 1 oz. 17 dwt. 12 gr. 



50 



40 



30 



20 



10 



Edge z 

 Middle m 

 Edge x 

 Edge z 

 Middle m 

 Edge x 

 Edge z 

 Middle m 

 Edge x 

 Edge z 

 Middle m 

 Edge x 

 Edge z 

 Middle m 

 Edge x 

 Edge z 

 Middle m 

 Edge x 

 Edge z 

 Middle m 

 Edge x 

 Middle m 



