Dr. Chowne on the Velocities of Currents of Air. 291 



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. 



Surface Condenser. — A three-horse power high-pressure steam- 

 engine was procured for our experiments. Wishing to give it equal 

 power with a lower pressure, we caused the steam from the eduction 

 port to pass downwards through a perpendicular iron gas-pipe, ten 

 feet long and an inch and a half in diameter, placed within a larger 

 pipe through which water was made to ascend. The lower end of 

 the gas-pipe was connected with the feed-pump of the boiler, a small 

 orifice being contrived in the pump cover in order to allow the 

 escape of air before it could ^lass, along with the condensed water, 

 into the boiler. This simple arrangement constituted a "surface 

 condenser" of a very efficient kind, giving a vacuum of 23 inches, 

 although considerable leakage of air took place, and the apparatus 

 generally was not so perfect as subsequent experience would have 

 enabled us to make it. Besides the ordinary well-known advantages 

 of the "surface condenser," such as the prevention of incrustation 

 of the boiler, there is one which may be especially remarked as 

 appertaining to the system we have adopted, of causing the current of 

 steam to move in an opposite direction to that of the water employed 

 to condense it. The refrigerating water may thus be made to pass 

 out of the condenser at a high temperature, while the vacuum is that 

 due to a low temperature ; and hence the quantity of water used for 

 the purpose of condensation may be materially reduced. We find 

 that our system does not require an amount of surface so great as to 

 involve a cumbrousness or cost which would prevent its general 

 adoption, and have no doubt that it will shortly supersede that at 

 the present time almost universally used. 



" Researches on the Velocities of Currents of Air in Vertical Tubes, 

 due to the presence of Aqueous Vapour in the Atmosphere." By 

 W. D. Chowne, M.D. 



This was a pa[)er supplementary to one presented June 14th, 1855, 

 an abstract of which was published in the ' Philosophical Magazine ' 

 for March, 1856. The author having ascertained that an upward 

 current of air becomes established in a vertical tube placed in as 

 quiescent an afmosj)herc as can be obtained, and having demonstrated 

 its existence by means of anemometric discs placed in tubes as de- 

 scribed in that pa[)er, jjroceeded to ascertain the velocity of the 

 currents by which the discs were moved. 



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