1887 .] 
Mr J. Aitken on Thermometer Screens. 
429 
siderably lower than those got with the bottom open. On dull and 
windy days the differences in the readings were not so great. 
To illustrate the influence of the weather on the readings of the 
Stevenson screen with open bottom, I shall use all the readings 
taken with the screens between the 23rd July and the 19th August 
of this year; these are arranged in the table (p. 428). 
In the first column of the table are the dates of the observa- 
tions ; in the second column the temperatures of the air as given 
by the C screen ; in the third column is the excess error of the 
Stevenson screen with bottom open; and in the last column is the 
estimated force of the wind, on a scale of from 0* to 10. It will 
be seen from the last column, that from the 23rd to the 29th July 
the weather was stormy, and that on the following days there was but 
little wind. The effect of these two conditions on the comparative 
readings of the screens is very evident. While the wind was strong 
it will be noticed that the difference between them was often less 
than one degree, but when the wind fell it was frequently more than 
two degrees. 
One object of meteorological observatories is to tell us something 
about the climate of the place — that is, something about its effects 
on animal and vegetable life. Now it is admitted by every one that 
the indications of the instruments in general use in our observatories 
do not by any means agree with the indications given by our 
bodies. The thermometer often says one thing, while our feelings 
indicate something quite different. No doubt, part of this disagree- 
ment is the result of the more or less healthy condition of our 
bodies at the time ; they are, so to speak, instruments with shifting 
scale. But apart from this, there is frequently a wide difference 
between the indications of the meteorological instruments and the 
average feeling of a great number of people. This results from the 
meteorological instruments not being affected by the same causes as 
our bodies. The thermometer may indicate that the air is warm, 
while we may feel it to be cold. This may be the result of the air 
being dry, and causing a cooling effect by a rapid evaporation from 
our bodies. We have therefore to check the readings of the dry bulb 
thermometer by a reference to the wet one ; we thus get a greater 
similarity between the indications of the meteorological instruments 
and those of our bodies. 
