418 Experimental Researches on the Depressions [July, 



and by manoeuvring the stopcocks (k, k'J at the two ends of the brass 

 tube, the pressure could be maintained at any point, and the draft of 

 air regulated until the temperature of the wet-bulb had been satisfac- 

 torily ascertained. 



Finding that the labour of working the pump was rather irksome 

 in a climate of 95°, I afterwards availed myself of the vacuum engine 

 of the coining-press room in the Mint to relieve me from this duty. 

 In the pipe leading from the twelve recoil-pumps of the presses a 

 vacuum of about (or rather 30 — 27) inches is constantly maintained 

 by the steam engine, so that by adapting the tube F to this with a 

 stopcock, I was enabled to regulate the pressure, and prolong each 

 interval with the utmost ease and comfort. 



It will be seen from the table of experiments below, that by em- 

 ploying a current of dry air the freezing point was readily attained 

 under a pressure of 7 J inches, while the dry thermometer, only one 

 inch from it, marked 92° : whereas all who have tried Leslie's pro- 

 cess for freezing have found it exceedingly difficult in the hot weather 

 of this country to produce ice with a vacuum nearly perfect. The 

 reason has been already explained : in the latter case the partially 

 moist atmosphere arrests the progress of refrigeration ; whereas in the 

 latter, the vapour rising from the evaporating surface is continually 

 removed ; — it is, in fact, like sitting under a punkah or without it, an 

 illustration that requires no comment to an Indian reader ! Of such 

 influence is the motion of the air in the experiment, that, as will be 

 seen presently, a cold much below the freezing point may be attained 

 under a pressure of 4~ inches, with common air at 92° containing six- 

 tenths of its vapour of saturation (dew-point = 75°) and without the 

 aid of sulphuric acid, or any other artificial means of previously drying 

 it ! This unexpected result opens a wide field for speculation as to the 

 possibility of modifying the apparatus of Leslie for the artificial 

 production of ice ; and I hope, when leisure permits, to resume the 

 thread of this collateral and highly interesting discovery. The nature 

 of the problem teaches us d, priori, that if a temperature of 20° can be 

 attained under a pressure of 4J inches, the cold at two inches ought 

 to be many degrees below zero of Fahrenheit's scale ! 



Out of four experiments made with the air-pump, and eight with the 

 Mint vacuum engine, it will be sufficient, after quoting the numerical 

 results of the whole, and referring to the accompanying diagram 

 (PI. XXII. fig. 10.) for a comprehensive view of their general bearing, 

 to select two or three of the most regular examples for analytical dis- 

 cussion. 



