I 



experienced by Air in rushing through small Apertures. 483 



equalizing the pressure of the air^) into one end of a spiral leaden 

 pipe 24 feet long and f^^ths of an inch in diameter, provided 

 with a stopcock at its other end. The spiral was in all the expe- 

 riments kept immersed in a large water-bath. 



In the first series of experiments, the temperature of the bath 

 was kept as nearly as possible the same as that of the surround- 

 ing atmosphere ; and the stop-cock, which was kept just above 

 the surface of the water, had a vulcanized india-rubber tube tied 

 to its mouth. The forcing-pump was worked uniformly, and 

 the stop-cock was kept so nearly closed as to sustain a pressure 

 of from two to five atmospheres within the spiral. A thermo- 

 meter placed in the vulcanized india-rubber tube, with its bulb 

 near the stop-cock, always showed a somewhat lower temperature 

 than another placed in the water-bath*; and it was concluded 

 that the air had expei-ienced a cooling efi"ect in passing through 

 the stop-cock. 



To diminish the efi"ects which might be anticipated from the 

 conduction of heat through the solid matter round the narrow 

 passage, a strong vulcanized india-rubber tube, a few inches long, 

 and of considerably less diameter than the former, was tied on 

 the mouth of the stop-cock in place of that one which was 

 removed, and tied over the mouth of the narrower. The stop- 

 cock was now kept wide open, and the naiTow passage was 

 obtained by squeezing the double india-rubber tube by means of 

 a pair of wooden pincers applied to compress the inner tube very 

 near its end, through the other surrounding it. The two ther- 

 mometers were placed, one, as before, in the bath, and the other 

 in the wide india-rubber tube, with its bulb let down so as to be 

 close to the end of the narrower one within. It was still found 

 that, the forcing-pump being worked as before, when the pincers 

 were applied so as to keep up a steady pressure of two atmospheres 

 or more in the spiral, the thermometer placed in the current of 

 air flo^ving from the narrow passage showed a lower temperature 

 than that of the air in the spiral, as shown by the other. Some- 



* When the forcing-i)imip is worked bo as to keep up a uniform pressure 

 in the spiral, and the water of the bath is stirred so as to be at a uniform 

 temperature tliroughout, tliis temperature will be, with almost perfeet accu- 

 racy, the temperature of the air as it approaches the stop-coek. It is to 

 be remarked, however, that when, by altering the ajierture of the stop-coek, 

 or the rate of working the jjunip, the ])ressure within the spiral is altered, 

 even although not very suddenly, the air throughout the spiral, up to the 

 narrow ])assage, alters in tem])erature on account of the expansion or con- 

 densation which it is experiencing, and there is an immediate corresponding 

 alteration in the temperature of the stream of air flowing from Ike riij)i<ls, 

 which j)roduces often a most sensible effect on the thermometer in the 

 issuing stream. 



212 



