252 Proceedings of Royal Society of Edinburgh. [sess. 
will not show the influence of humidity, so markedly as those made 
in warm climates. 
In making out the table given with this paper, the humidity 
might have been shown in other ways. For instance, in place of 
entering the wet-bulb depression, the relative humidity or the 
dew-point of the air might have been given. It was, however, 
thought better to enter the wet-bulb observations themselves, since 
either of the others can be easily calculated from them. At 
ordinary temperatures the relative humidity and the dew-point are 
approximately proportional to the depression of the wet-bulb, the 
dew-point being nearly twice as much as the wet-bulb below the 
temperature of the air. But for low temperatures the proportions do 
not agree so well. For instance, the dew-point at the temperature 
of 60° is 1 ‘88 times the wet-bulb depression below the temperature 
of the air; at 50° it is 2*06 times ; while if the temperature is 34°, 
it is 2*77 times. So that for low temperatures part of the clearness 
is due not only to the lower vapour pressure, but also because, for a 
given wet-bulb depression, the dew-point is really farther below the 
temperature of the air with a low than with a high temperature. 
For instance, taking the above examples with a wet-bulb depression 
of 4°, the dew-point at 60° is 7° 5 below the temperature of the air, 
at 50° it is 8° *2, while at 34° it is fully 11°| and when the tempera- 
tures are still lower the difference increases rapidly. The result 
is that, when there is a small difference between the wet and dry 
bulbs while the temperature is very low, the dew-point is a con- 
siderable distance below the temperature of the air. For instance, 
at a temperature of 20° and a wet-bulb depression of only 1°, the 
dew-point is fully 8° below the temperature of the air. If the 
temperature had been 53°, in order that the dew-point might be as 
many degrees below the temperature of the air, the wet-bulb 
depression would require to be 4°. 
We must therefore only use the wet-bulb depression for com- 
parison when the temperature is over 50°, below that temperature 
allowance must be made. In the future, perhaps, either the depres- 
sion of the dew-point or the relative humidity would be a better 
figure to use in these tables. 
Returning now to the table in the appendix, it will be observed 
that on the 24th that there were 725 particles per c.c. and a depres- 
