562 



On inspecting the above Table, it will be seen that tlie 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. In 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. 



The thermal effects on different sides of a sphere moving through 

 air, have been investigated by us experimentally by whirling a thin 

 glass globe of 3' 58 inches diameter along with the smaller ther- 

 mometer, the bulb of which was placed successively in three positions, 

 viz. in front, at one side, and in the rear. In each situation it was 

 placed as near the glass globe as possible without actually touching it. 



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

 increased resistance. 



