74 Proceedings of Royal Society of Edinburgh, [jan. 17 , 
till 11.45, at which hour there was a difference of 0°‘9 between 
screen C and the Stevenson. At 12.5 the difference was as much 
as 1°'2, after which it fell to about l o, 0 at 12.30; after this hour the 
weather changed, the sky clouded all over, the radiation effect gradually 
fell, and the readings having no further interest were stopped. 
The following day, the 18th, being a most perfect day, the trials 
were continued. On this occasion the sun rose in a cloudless sky, and 
shone brightly till it set. The sun was warmer, and the wind less 
than on the previous day. All the conditions were thus favourable 
for a trial of this kind, as they tended to bring out in a marked 
manner any differences due to radiation. As on the previous day, 
the readings were taken at five-minute intervals, with two short 
breaks. They were begun at 10.15 a.m., and continued till 
3.15 p.m.; the temperatures are all shown in the lowest series of 
curves in PI. IV. It will be seen that on this occasion the 
dilference was often more than l o, 0, and attained a maximum 
of 1 ° *85 at 2.20 p.m. The wide separation of the curves at 
this hour was due to the wind dying quite away, so that, though 
the louvres were not exposed to so strong a radiation at that hour 
as they had been at an earlier part of the day, yet they got more 
highly heated, as there was no wind to cool them. 
It may be mentioned that the fine silvered bulb was also on trial 
on these two days. It is, however, impossible to enter its readings 
on PL IV., as they were almost exactly the same as screen C, scarcely 
ever varying from it more than 0°*1 ; only once it was observed to 
vary 0 o, 2, but generally the two readings were the same. We may, 
therefore look on the readings given by the C screen as nearly 
correct on those days, and those given by the Stevenson as too high. 
We now come to the consideration of the cause of this very great 
error in the Stevenson screen, so late in the year, when radiation is 
so much reduced. As the error is principally due to the heating of 
the louvres by solar radiation, and as the temperature to which 
they are raised depends on the amount of heat received by them, 
and the rate at which this heat is carried away by the air, we shall 
consider these two points separately. First, as to the radiation. 
On the two days on which these trials were made, two different 
kinds of radiation thermometers were exposed to the sunshine. 
One of them was an ordinary vacuum black-bulb thermometer, 
