482 Roijal Society. 



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 mthout actually touching it. 



Table VIII. — Smaller Thermometer whirled along with glass globe. 



Rise in divisions of scale. 



Velocity in feet , " > 



per second. Therm, in front. Tiierm. at side. Therm, in rear. 



10 4 



40 10-5 



..... 78 51 



90 71-7 



90 68 



112 76 



70-92 107 



The effects of fluid friction are strikingly evident in the above 

 results, particularly at the slow velocities of 3 and 7 feet per second. 

 It is clear from these, that the air, after coming in contact with the 

 front of the globe, traverses with friction the equatorial parts, giving 

 out an accumulating thermal effect, a part of which is carried round 

 to the after pole. At higher velocities the effects of friction seem 

 rapidly to diminish, so that at velocities between 23 and 38 feet per 

 second, the mean indication of thermometers placed all round the 

 globe would be nearly constant. Our anticipation (written before 

 these latter experiments were made), that a complete verification of 

 the theory propounded at the commencement was impossible with 

 our present means, is thus completely justified. 



It may be proper to observe, that in the form of experiment 

 hitherto adopted by us, the results are probably, to a trifling extent, 

 influenced by the vortex of air occasioned by the circular motion. 



We have on several occasions noticed the effect of sudden changes 

 in the force of wind on the temperature of a thermometer held in it. 

 Sometimes the thermometer was observed to rise, at other times to 

 fall, when a gust came suddenly on. "When a rise occurred, it was 

 seldom equivalent to the eff"ect, as ascertained by the foregoing ex- 

 periments, due to the increased velocity of the air. Hence we draw 

 the conclusion, that the actual temperature of a gust of wind is lower 

 than that of the subsequent lull. This is probably owing to the air 

 in the latter case having had its vis viva converted into heat by 

 collision with material objects. In fact we find that in sheltered 

 situations, such for instance as one or two inches above a wall op- 

 posite to the wind, the thermometer indicates a higher temperature 

 than it does when exposed to the blast. The question, which is one 

 of great interest for meteorological science, has hitherto been only 

 partially discussed by us, and for its complete solution will require a 

 careful estimate of the temperature of the earth's surface, of the 

 effects of radiation, &c., and also a knowledge of the causes of gusts 

 in different winds. 



