184 Proceedings of Eoyal Society of Edinhurgli. [sess.. 
calculating vapour pressure, relative humidity, and dew-point from 
dry and wet bulb observations agree very badly — indeed, at times 
disagree very remarkably. I have reduced some of my observations 
by means of Guyot’s Smithsonian Tables (last edition) as well as 
those of Mr Glaisher. Here are examples of the differences that 
may occur (the temperatures are expressed in degrees Fahrenheit): — 
Dry. 
Wet. 
Differ- 
ence. 
Vapour Pressure. 
Relative Humidity. 
Dew-point. 
Glaisher. 
Smith- 
sonian. 
Glaisher. 
Smith- 
sonian. 
Glaisher. 
Smith- 
sonian. 
20-2 
17-3 
2-9 
•037 
•062 
34 
55 
- 3-3 
8-0 
20-3 
19-4 
0-9 
•078 
•094 
73 
87 
13-1 
17-0 
36-8 
30-0 
6-8 
•109 
•084 
50 
41 
20-3 
14-5 
42-7 
36*5 
62 
•ICO 
•135 
59 
49 
1 29-1 
1 25-0 
Considering the few experiments made when testing the apparatus^ 
at low levels, the results appear to agree fairly well with those of 
Glaisher’s Tables, but the number of these is not sufficient to 
warrant any definite statement. 
The need for additional experiments is obvious. Eesults at 
summer temperatures are wanted to complete the diagram ; and, to 
determine how far pressure influences the other factors, a series of 
simultaneous experiments must be made at two extreme stations 
at least, and, if possible, also at an intermediate one. To ensure 
perfect comparability of data, special precautions must be taken, 
for instance, by having the dry and wet bulbs always in analogous 
conditions by using some aspiration psychrometer, and it would be 
Avell at the same time to record the readings of other instruments 
used for determining the hygrometric conditions of the atmosphere. 
In conclusion, I would thank the Directors of the Ben Nevis 
Observatory for giving every facility for carrying out these experi- 
ments, and to the observatory staff, especially to Messrs Omond and 
Eankin, for their help, without which the number of the experiments 
would have been much less. I am also especially indebted to 
Professor Tait and Dr Buchan, who have aided me in every way in 
planning and discussing the results of this research ; and I thank 
Mr James Wood for helping me with the calculations. 
