248 



PHILOSOPHICAL TRANSACTIONS. 



[anno 1798. 



I. The Bakerian Lecture. Being Experiments on the Resistance of Bodies moving 

 in Fluids. By the Rev. S. Vince y A.M.y F. R. S., fife, Anno 17 98. 

 Vol. LXXXV1I1. p. I. 



In a former paper on the motion of fluids, I stated the difficulties to which the 

 theory is subject, and showed its insufficiency to determine the time of emptying 

 vessels, even in the most simple cases ; I also proved, by actual experiments, that 

 in many instances there was no agreement between their results and those deduced 

 from theory. The great difference between the experimental and theoretical con- 

 clusions, in most of the cases which respect the times in which vessels empty them- 

 selves through pipes, necessarily leads us to suspect the truth of the theory of the 

 action of fluids under all other circumstances. In the doctrine of the resistances of 

 fluids, we see strong reasons to induce us to believe, that the theory cannot gene- 

 rally lead us to any true conclusions. When a body moves in a fluid, its particles 

 are struck by the body ; and in our theoretical considerations, after this action, the 

 particles are supposed to produce no further effect, but are conceived to be as it 

 were annihilated. But in fact this cannot be the case ; and what we are to allow for 

 their effect afterwards, is beyond the reach of mere theoretical investigation. What- 

 ever theory therefore we can admit, must be that which is founded on such experi- 

 ments as include in them every principle which is subject to any degree of uncertainty. 

 We must therefore have recourse to experiments, in order to establish any conclu- 

 sions on which we may afterwards reason. In the paper above mentioned, I de- 

 scribed a machine to find the resistances of bodies moving in fluids ; since which 

 time, I have made a variety of experiments with it, on bodies moving both in air 

 and water, and I have every reason to be satisfied of its great accuracy. In this 

 paper, I propose to examine the resistance which arises from the action of non- 

 elastic fluids on bodies. 



This subject divides into 2 parts : we may consider the action of water at rest on 

 a body moving in it, or we may consider the action of the water in motion on the 

 body at rest. We shall first give the result of our experiments in the former case, 

 and compare them with the conclusions deduced from theory. Now the radius of 

 the axis of the machine used in these experiments was 0.2117 in. the area of the 4 

 planes was 3.73 in. the distance of their centres of resistance from the axis was 

 7.57 in. and they moved with a velocity of 0.66 feet in a second. The first column 

 of the following table exhibits the angles at which 

 the planes struck, the fluid ; the 2d column shows 

 the resistance by experiment, in the direction of 

 their motion, in Troy ounces ; the 3d column gives 

 the resistance by theory, assuming the perpendi- 

 cular resistance to be the same as by experiment ; 

 the 4th column shows the power of the sine of 

 the angle to which the resistance is proportional. 



