of Edinburgh, Session 1885—86. 
959 
almost perfectly ; while at Ben Nevis the hygrometer tended to give 
higher results. As a rule the differences do not exceed the limits of 
errors of observation, and are only shown by the general direction 
of the lines in the diagram. 
The expression given by Dr Apjohn is of the form 
j j 30 > 
where f" is the vapour pressure corresponding to the temperature of 
the dew-point, /' that to the temperature of the wet bulb, S is 
difference of temperature between wet and dry, P the atmospheric 
pressure at the time of observation, 30 being mean sea-level pressure, 
and m a coefficient depending on the specific heat of air and the 
latent heat of aqueous vapour. Dr Apjohn communicated the 
details of the investigation which led him to the above formula to 
the Royal Irish Academy, and the paper was published in the 
Transactions in 1834. He afterwards verified his results by experi- 
mental investigations (Trans. R. I. A. ; Phil. Mag., 1835, vii. p. 
266; Brit. Assoc. Rep, 1843, p. 36). In the course of these, by 
heating air, of which the point of saturation was known, he obtained 
very great differences of wet and dry, and found his formula to give 
accurate results. 
Without discussing objections to Apjohn’s methods (for which 
see his original paper, also Clerk-Maxwell in Ency. Brit., art. 
“ Diffusion,” 9th edition), we may simply take the form given with 
the value ~ T for m. Putting into the diagram lines formed by 
taking fixed values of /', and increasing 8, the value of P being 
as 25 ‘300 throughout, being the mean barometric pressure for the 
time over which the observations extend, we get a system of parallel 
straight lines. Comparing these, as before, with the hygrometer 
lines, we find that at high temperature of wet bulb the dew-points 
ascertained by Apjohn’s formula are on the whole too low, i.e ., the 
lines representing them are slightly steeper than the hygrometer lines. 
As the wet bulb temperature is lowered the differences between the 
two results diminish, and with wet bulb readings between 35° F. and 
40° F. Apjohn’s formula agrees with the hygrometer within limits 
of observational error, even .with differences of wet and dry amount- 
ing to over 10° F. When we come to temperatures below 32° F., 
