Prof. Thomson on the Thermal Effects of Fluids in Motion. 289 



such a velocity would be from 287° to 268° absolute temperature, 

 or 19° Cent. 



The effects of fluid friction in different parts of the stream would 

 require to be known in order to estimate the reduced velocity in any 

 narrow part, according to either the density on the high-pressure 

 side or the density on the low-pressure side. We have not as yet 

 made any sufficient investigation to allow us to give even a conjec- 

 tural estimate of what these effects may be in any case. But it 

 appears improbable that the " reduced velocity," according to the 

 density on the high-pressure side, could ever with friction exceed 

 the greatest amount it could possibly have without friction. It 

 therefore seems improbable that the "reduced velocity" in terms of 

 the density on the high-pressure side can ever, in the narrowest part 

 of the channel, exceed 644 feet per second, if the temperature of the 

 high-pressure air moving slowly be about the atmospheric tempera- 

 ture of 13° Cent, used in the preceding estimate. 



Experiments in which we have forced air through apertures of 

 TWo> TWO' aiid rffoths of an inch in diameter drilled in thin 

 plates of copper, have given us a maximum velocity reduced to the 

 density of the high-pressure side equal to 550 feet per second. But 

 there can be little doubt that the stream of air, after issuing from an 

 orifice in a thin plate, contracts as that of water does under similar 

 circumstances. If the velocity were calculated from the area of this 

 contracted part of the stream, it is highly probable that the maxi- 

 mum velocity reduced to the density on the high-pressure side would 

 be found as near 644 feet as the degree of accuracy of the experi- 

 ments warrants us to expect. 



As an example of the results we have obtained on examining the 

 temperature of the rushing stream by a thermo-electric junction 

 placed ith of an inch above the orifice, we cite an experiment, in 

 which the total pressure of the air in the receiver being 98 inches of 

 mercury, we found the velocity in the orifice equal to 535 and 1780 

 feet respectively as reduced to the density on the high-pressure and 

 that on the atmospheric side. The actual velocity in the small 

 aperture must have been greater than either of these, perhaps not 

 much greater than 1 780, the velocity reduced to atmospheric den- 

 sity. If it had been only this, the cooUng effect would have been 



exactly T -^- /iir^V , that is, a lowering of temperature amount- 

 ing to 150^^ Cent. But the amount of cooling effect observed in the 

 experiment was only 13° Cent.; nor have we ever succeeded in ob- 

 serving (whether with thermometers held in various positions in the 

 stream, or with a thermo-electric arrangement constituted by a nar- 

 row tube through which the air flows, or by a straight wire of two 

 different metals in the axis of the stream, with the junction in the 

 place of most rapid motion, and in other positions on each side 

 of it,) a greater cooling effect than 20° Cent ; we therefore infer 

 that a body round which ah' is fiowiny raindly acquires a hiyhcr 

 temperature than the average temperature of the air close to it all 

 round. The explanation of this conclusion probably is, that the 

 Phil. May, S. 4. Vol. 13. No. 86. April 1857. X 



