98 ^ Dr. Mason's Description 



Leslie concluded, that it was supplied entirely by the sur- 

 rounding medium ; Dr. Anderson asserts that it is derived 

 entirely from the water itself; while there are others who 

 maintain that it is at first derived from the water, until 

 refrigeration has attained its maximum intensity, and after- 

 wards from the atmosphere. 



Without entering into any discussion respecting the 

 merits of each of these theories, which would be incon- 

 sistent with the length of this article ; the most rational 

 opinion appears to be that of Sir J. Leslie, namely, that it 

 is derived entirely from the surrounding medium ; for were 

 it derived from the water, and no compensation made, 

 there could be no limits to refrigeration so long as moisture 

 was supplied to the bulb, and evaporation continued. 



The method by which the moistened bulb of a thermo- 

 meter becomes reduced in temperature, appears to me to be 

 the following. 



Both bulbs while dry continue to receive equal incre- 

 ments of heat from the surrounding medium. We will 

 suppose them both at 70° Fahrenheit, when water at the 

 same temperature is applied to one of the bulbs, and the 

 atmosphere not saturated with humidity : the existing 

 vapour having a final tension of 0-657, corresponding to a 

 temperature of 65°, at which full saturation would take 

 place. 



Evaporation will immediately commence : a portion of 

 the water will enter into the state of vapour having a ten- 

 sion of 0*770 corresponding to a temperature of 70°, hence 

 the bulb will be reduced in temperature ; that portion of 

 atmospheric air which surrounds it will become cooled in 

 like proportion, its heat being expended in converting 

 another portion of water into vapour, and thus its becoming 

 latent will not affect the temperature of the thermometric 

 fluid. 



Evaporation continuing, the next portion of fluid con- 

 verted into vapour will have a less tension corresponding to 

 the reduction of temperature ; the air in immediate contact 

 with the bulb will be reduced in temperature in the same 

 proportion, converting another portion of water into 

 vapour. 



These two processes will go on, the one gradually de- 



