On the Thermal Effects of Fluids in Motion. 479 



spheroid of revolution, moving in the direction of its axis, would 

 experience at its equator a depression of temperature, greater if it be 

 an oblate spheroid, or less if it be a prolate spheroid, than f of 

 the elevation of temperature at each pole. 



It must be borne in mind, that, besides the hmitation to velocities 

 of the body small in comparison with the velocity of sound, these 

 conclusions involve the supposition that the relative motions of the dif- 

 ferent parts of the air are unresisted by mutual friction, a supposition 

 which is not even approximately true in most cases that can come 

 under observation. Even in the case of a ball pendulum vibrating 

 in air. Professor Stokes* finds that the motion is seriously influenced 

 by fluid friction. Hence with velocities which could give any effect 

 sensible on even the most deUcate of the ether thermometers yet 

 made (330 divisions to a degree), it is not to be expected that any- 

 thing like a complete verification or even illustration of the preceding 

 theory, involving the assumption of no friction, can be had. It is 

 probable that the forward polar region of heating effect will, in con- 

 sequence of fluid friction, become gradually larger as the velocity is 

 increased, until it spreads over the whole equatorial region, and does 

 away with all cooling effects. 



Our experimental inquiry has hitherto been chiefly directed to 

 ascertain the law of the thermal effect upon a thermometer rapidly 

 whirled in the air. We have also made some experiments on the 

 modifying efi"ects of resisting envelopes, and on the temperatures at 

 difi'erent parts of the surface of a whirled globe. The whirling ap- 

 paratus consisted of a wheel worked by hand, communicating rapid 

 rotation to an axle, at the extremity of which an arm carrying the 

 thermometer with its bulb outwards was fixed. The distance between 

 the centre of the axle and the thermometer bulb was in all the expe- 

 riments 39 inches. The thermometers made use of were filled with 

 ether or chloroform, and had, the smaller 275, and the larger 330 

 divisions to the degree Centigrade. The lengths of the cylindrical 

 bulbs were ^ and l^^ inch, their diameters '26 and '48 of an mch 

 respectively. 



Table I. — Small bulb Thermometer. 

 Velocity in feet Rise of temperature ia Rise divided by 



per second. divisions of the scale. square of velocity. 



46-9 27i -0125 



51-5 32 -0121 



68-1 46i -0100 



72-7 57^ -0109 



78-7 m -0109 



84-8 74 -0103 



104-5 91 -0083 



130-2 151 -0089 



133-2 172 -0097 



145-4 191 -0090 



Mean.. -01020 



• " On the Effect of the Internal Friction of Fluids on the Motion of Pen- 

 dulums," read to the Cainliridgc Philosophical Society, Dec. 9, 1850, and pub- 

 lished in vol. ix. part 2 of their Transactions. 



