290 Royal Society :■ — 



surface of coutact between the air and the solid is the locality of th^ 

 most intense frictional generation of heat that takes place, and that 

 consequently a stratum of air round the body has a higher average 

 temperature than the air further off; but whatever the explanation 

 may be, it appears certainly demonstrated that the air does not give 

 its own temperature even to a tube through which it flows, or to a 

 wire or thermometer-bulb completely surrounded by it. 



Having been convinced of this conclusion by experiments on rapid 

 motion of air through small passages, we inferred of course that the 

 same phenomenon must take place universally whenever air flows 

 against a solid or a solid is carried through air. If a velocity of 

 1780 feet per second in the foregoing experiment gave 137° Cent, 

 difference of temperature between the air and the solid, how probable 

 is it that meteors moving at from six to thirty miles per second even 

 through a rarefied atmosphere, really acquire, in accordance with 

 the same law, all the heat which they manifest ! On the other hand, 

 it seemed worth while to look for the same kind of effect on a much 

 smaller scale in bodies moving at moderate velocities through the 

 ordinary atmosphere. Accordingly, although it has been a practice 

 in general undoubtingly followed, to whirl a thermometer through 

 the air for the purpose of finding the atmospheric temperature, we 

 have tried and found, vnth thermometers of different sizes and va- 

 riously shaped bulbs, whirled through the air at the end of a string, 

 ■with velocities of from 80 to 120 feet per second, temperatures 

 always higher than when the same thermometers are whirled in 

 exactly the same circumstances at smaller velocities. By alternately 

 whirling the same thermometers for half a minute or so fast, and 

 then for a similar time slow, we have found differences of tempera- 

 ture sometimes little if at all short of a Fahrenheit degree. By 

 whirling a thermo-electric junction alternately fast and slow, the 

 same phenomenon is most satisfactorily and strikingly exhibited by 

 a galvanometer. This last experiment we have performed at night, 

 under a cloudy sky, with the galvanometer within doors, and the 

 testing thermo-electric apparatus whirled in the middle of a field ; 

 and thus, with as little as can be conceived of disturbing circum- 

 stances, we confirmed the result we had previously found by whirling 

 thermometers. 



Velocity of Air escaping through narrow Apertures. 

 In the foregoing part of this communication, referring to the cir- 

 cumstances of certain experiments, we have stated our opinion that 

 the velocity of atmospheric air impelled through narrow orifices was, 

 in the narrowest part of the stream, greater than the reduced velocity 

 corresponding to the atmospheric pressure ; in other words, that the 

 density of the air, kept at a constant temperature, was, in the narrow- 

 est part, less than the atmospheric density. In order to avoid mis- 

 conception, we now add, that this holds true only when the difference 

 of pressures on the two sides is small, and friction plays but a small 

 part in bringing down the velocity of the exit stream. If there is a 

 great difference between the pressures on the two sides, the reduced 



