On the Thermal Effects of Fluids in Motion. 481 



the constant velocity of 90 feet per second, the mean result did not 

 appear to indicate any decided difference of thermal effect. 



Another quarter of paper was now removed from the opposite 

 side. Then on whirhng so that the bared parts were anterior and 

 posterior, we obtained a rise of 83 divisions with a velocity of 93-8. 

 But on turning the thermometer on its axis one quarter round, so 

 that the bared parts were on each side, we found the somewhat 

 smaller rise of G2 divisions for a velocity of 90'8 feet per second. 



The effect of surface friction having been exhibited at slow velo- 

 cities with the })apered bulb, we were induced to try the effect of 

 increasing it by wrapping iron wire round the bulb. 

 Table YI.— Larger bulb Thermometer wrapped with iron wire. 

 Velocity in feet Rise in di\'isions Rise divided by 



per second. of scale, square of velocity. 



15-36 10-25 -0434 



2304 33 -0623 



30-71* 49-25 -0522 



46-08 68-75 '0324 



6912 98 -0206 



111-34 185 -0149 



126-72 207 -0129 



153-55 above 280 above -0118 



On inspecting the above Table, it will be seen that the thermal 

 effect produced at slow velocities was five times as great as with the 

 bare bulb. This increase is evidently due to friction. In fact, as 

 one layer of wire was employed, and the coils were not so close as to 

 prevent the access of air between them, the surface must have been 

 about four times as great as that of the uncovered bulb. At high 

 velocities, it is probable that a cushion of air which has not time to 

 escape past resisting obstacles makes the actual friction almost inde- 

 pendent of variations of surface, which leave the magnitude of the 

 body unaltered. lu conformity with this observation, it will be seen 

 that at high velocities the thermal effect was nearly reduced to the 

 quantity observed with the uncovered bulb. Similar remarks apply 

 to the following results obtained after wrapping round the bulb a fine 

 spiral of thin brass wire. 



Table VII. — Bulb wrapped with a spiral of fine brass wire. 

 Velocity in feet Rise in divisions Rise divided by 



per second. of scale. square of velocity. 



7-68 2-5 -0424 



15-36 13-5 -0572 



23-04 36-5 -0687 



30-71 48 -0509 



46-08 64-5 -0304 



76-8 103-5 -0175 



115-18 224-5 -0169 



148-78 264 -0119 



The thermal effects on different sides of a sphere moving through 

 • Tlie whirring sound began at this velocity. According to its intensity tlie 

 thermal effect must necessarily suffer diminution ; unless indeed it gives rise to 

 increased r istancc. 



