184 



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, r 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 

 velocity will, on the contrary, be less than that corresponding with 

 the atmospheric pressure ; and even if the pressure in the most rapid 

 part falls short of the atmospheric pressure, the density may, on ac- 

 count of the cooling experienced, exceed the atmospheric density. 



We stated that, at 57 Fahr., the greatest velocity of air passing 

 through a small orifice is 550 feet per second, if reduced to the 

 density on the high-pressure side. The experiments from which we 

 obtained this result enable us also to say that this maximum occurs, 

 with the above temperature and a barometric pressure of 30' 14 inches, 

 when the pressure of the air is equal to about 50 inches of mercury 

 above the atmospheric pressure. At a higher or lower pressure, a 

 smaller volume of the compressed air escapes in a given time. 

 * Received June 19, 1856. 



