MR. W. N. SHAW ON HYGROMETRIC METHODS. 



101 



average in two hours, giving a minimum motion cf 3 cm. per minute assuming, that 

 is, that the motion was uniform over the whole central section of the globe. The 

 motion at the thimble in the middle of the globe was probably considerably greater 

 than this, but at any rate could not be called rapid. Under such circumstances the 

 temperature of the thimble and ether does not rise uniformly, so that one had to keep 

 the ether mixed by an occasional bubble of air, and the practical temperature limits of 

 deposit and no deposit, though sometimes two-tenths, were sometimes four-tenths of 

 a degree centigrade. 



18. Let us call the temperature of saturation of the air the theoretical dew-point. 

 Then, in order that the observed dew-point may coincide with the theoretical, the 

 following assumptions seem to be required : (1.) That as soon as the temperature of 

 the thimble is below the theoretical dew-point a deposit of moisture is formed. 

 (2.) That the observer can see it. (3.) That there is no deposit when the temperature 

 is above the theoretical dew-point. These three assumptions may be regarded as 

 independent, though their consideration must to some extent overlap. I will take 

 No. 2 first. The possibility of seeing a deposit depends upon circumstances. In the 

 first place the thimble must be highly polished. In one observation, No. 15, when the 

 thimble was tarnished, the saturator being at 16'96 C., and the chemical method corre- 

 sponding to a temperature of 17 '2 C., two readings of temperature of earliest visible 

 deposit were 15 C. and 16 '3 C. respectively, either of them much lower than the 

 theoreticid dew-point, and indicating uncertainty in the readings, which did not occur 

 when the thimble was well polished. Secondly, the illumination is of importance. On 

 this point no general direction seems useful ; my observations were made indoors, in a 

 room with two windows. I found that I could see the deposit most easily sitting 

 with my back to the windows, and having a reflector or a lamp placed almost behind 

 the thimble, so that the light grazed the side of it, a uniform dark background being 

 immediately behind. Small specks on the polished surface seem to facilitate the 

 reading, the first indication of a deposit being a slight fringe round the specks. 

 Thirdly, different observers do not take the same readings, although practice may do 

 away with the differences. 



I was assisted in the observations by one of the students at the Cavendish Labora- 

 tory, who had had no previous experience in dew-point readings ; at first our readings 

 dirtered considerably, mine being the higher. The following cases show this : 



