92 



MR. W. N. SHAW ON HYGROMETRIC METHODS. 



TABLE IV. 



It then occurred to me that the effect was probably due to differences of temperature 

 in different parts of the bath. 



I had not paid much attention to the uniformity of temperature, as the air was 

 drawn from a point in the interior of the copper cylinder quite close to the bulb of 

 the thermometer, and I considered that the air so withdrawn would be saturated at 

 that temperature, but I had neglected to take sufficient notice of the fact that it was 

 saturated air I was dealing with, and that the effect of the tube which led away 

 the air ought to be considered. It seemed likely that if the water in the bath was 

 not kept thoroughly well stirred, the water, cooled by the melting of the ice, would 

 sink rapidly until the temperature got near 4, and that then a layer of cold water 

 would be formed on the top of the bath, and cool the top of the cylinder and the 

 tube through which the air came ; if this was colder than the saturated air, a part of 

 the moisture would be deposited in the tube ; the layer of moisture thus formed 

 would, on the other hand, be at a higher temperature than the rest when the bath 

 gradually got warmer by contact with the air at the top, and the air for the experi- 

 ment would be slightly warmed and receive an addition to its moisture in passing over 

 this band of moisture. At the end of the series of observations there was no moisture 

 visible on the parts of the tube above the cylinder, but on drawing the tube out of its 

 cork a little way a collar of moisture was distinctly visible where the tube had been 

 cooled by conduction through the cork, and thus my suspicion was confirmed. In 

 consequence of this discovery, considerable attention was devoted to the stirring of 

 the bath, to keep it at a uniform temperature. During an observation I could not 

 hope that the tube would be kept free from deposit, and the only thing to do was to 

 prevent its having any effect on the temperature of saturation. The next six 

 observations show the result of this precaution ; the temperature began at 6'44 C., was 

 reduced to 3'lO C., and then allowed to rise to 7'93 C., and therefore included that 

 part of the scale in which the differences were previously most conspicuous. 



