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than has yet been obtained. As long as the bulb can be 
supplied with water this is constantly flowing up the thread, 
where it soon attains the temperature of evaporation, and 
then a comparatively equal film of water is always main- 
tained in the muslin surrounding the bulb, but as soon as 
the temperature is below freezing the personal equation 
plays a large part, for now we shall find that the covering 
of the bulb is a very material point, as well as the thickness 
of the ice sheet, which is no longer automaf ically arranged, 
and further, the temperature of the wet bulb after an ice 
sheet has been formed will vary very much from time to 
time. The instructions usually given are that the bulb 
should be wetted about half an hour before an observation, 
but I find that in practice many observers only do this a 
quarter of an hour before. I have already called attention 
to this not being long enough, and although I have not 
this winter obtained as striking results as I did some winters 
ago, yet through some hundred of observations, in which 
special attention was paid to the dew point, as that cannot 
change so rapidly or irregularly as the temperature, I am 
able to fully confirm what I then wrote as to a longer time 
being required. This is most important in relation to the 
Davos climate, because it has been charged against Davos 
that it is not dry, and while the relative moisture is no 
doubt greater than the popular talk about the place would 
lead us to expect, yet I think that many of the figures 
which have been published would make it appear moister 
than is really the case. 
The first observations were made by Dr. Spengler from 
1867 to 1871. These were made in a metal box attached to 
the side of the house, as is usually done in Switzerland, and 
as I felt somewhat doubtful as to the results obtained in a 
metal box so near a house, I put up, in 1870, a very large 
screen consisting of an inner box with single louvres, sur- 
rounded by an outer one at a distance of six inches, also 
