On the Rejijlance of Bodies moving in Fluids, 



507 



dIfFerence between the experimental vaA theoretical conclufions, in mofl of the cafes which 

 tcfpcft the times in which veffels empty themfclves through pipes> neceffarily leads us to 

 fufpett the truth of the theory of the a£lion of fluids under all other circumftances. In 

 the do6lrine of the refiftance of fluids, we fee ftrong reafons to induce us to beKeve> that 

 the theory cannot generally lead us to any true conclufions. When a body moves in a 

 fluid, its particles ftrike the body % and in our theoretical confi derations after this aftion, 

 the particles are fuppofed to produce no further effeft, but are conceived to be, as it were, 

 annihilated. But in faft this cannot be the cafe j and what we are to allow for their efFeiSb 

 afterwards, is beyond the reach of mere theoretical inveftigation. Whatever theory there- 

 fore we can admit, muft be that which is founded upon fuch experiments, as include in 

 them every principle which is fubjecl to any degree of uncertainty : we muft therefore 

 have recourfe to experiments, in order to eftablifh any conclufions upon which we may 

 afterwards reafon. In the paper above mentioned, I defcribed a machine to find the re- 

 fiftances of bodies moving in fluids ; fincc which time I have made a variety of experiments 

 with it upon bodies moving both in air and water, and have every reafon to b& fatisfied of 

 its great accuracy. In this paper I propofe to examine the refiftance which arifes from 

 the action of non-elaftic fluids upon bodies. 



This fubje£t divides itfelf into two parts; we may conGder the a£lIon of water at reft' 

 upon a body moving in it ; or we may confider the a£lion of the water in motion when- 

 the body is at reft. We will firft give the refult of our experiments in the former cafe,, 

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

 of the machine made ufe of, was 0,2117 inches; the area of the four planes, was 3,73 

 inches ; the diftances of their centres of refiftance from the axis, was 7,57 inches ; and 

 they moved with a velocity of 0,66 feet in a fecond. The firft column of the following 

 table exhibits the angles at which the planes ftruck the fluid }. the fecond column fhews 

 the refiftance by experiment, in the direftion of their motion in troy ounces : the third 

 column gives the refiftance by theory, afTuming the perpendicular refiftance to be the fame 

 as by experiment ; the fourth column fhews the power of the fine of the angle to which 

 the refiftance is proportional. 



The 



