1887.] Mr John Aitken on Thermometer Screens. 69 
all the instruments ought to he taken at the same moment, owing 
to the constant changes of temperature in the air. In practice it is 
therefore desirable to make the number of thermometers to be read as 
few as possible. The silvered bulb was, however, again put on trial, 
and as before, it gave readings much below the Stevenson screen; 
considering the season, its readings were as much below the screen 
as was observed in the first trials already mentioned. 
On 21st September a number of readings were taken from time 
to time during most of the day with the two Stevenson screens, the 
bottom of A being closed, while B was open. These readings are 
marked off at the top of PI. III., and the different readings connected 
by straight lines. The day was fine, with passing clouds, a little 
wind, and radiation fairly strong for the time of the year. It will 
he observed, that till after mid-day there was but little difference in 
the readings of the two screens ; this was owing to there being but 
little radiation before that hour. At a little before 12 o’clock the 
open screen was only 0 o, 3 above the closed one. A little before 1 
o’clock the bottom was taken out of A, and by 1 o’clock both 
screens read nearly alike. 
The bottom was again put in A, and when the next reading was 
taken at 1.15 the open screen read 0 o, 5 higher than the closed one, 
and during the whole day the open one gave the highest readings, 
the amount varying according to the radiation at the time. The black 
parts at the top of the diagram represent sunshine; they cannot, how- 
ever, be very correct, as they are drawn from the notes taken at the 
hour the readings were made, and thus only represent the condition 
of matters at that time, no intermediate record being taken. A 
sunshine recorder would have enabled me to put in these curves 
more correctly. The general result is, however, very easily seen from 
the record given. It will be noticed, that whenever there was sun- 
shine the open screen read much higher than the closed one, and that 
during the absence of sunshine they tended to read alike, but it was 
not till after radiation had entirely ceased that they read quite alike. 
At the 2 o’clock and the 2.5 readings the open screen was 0 o- 9 higher 
than the other, while a little before 3 o’clock the difference was as 
much as 1°*1. 
While this trial was in progress, in addition to the thermometer 
placed where the wet bulb usually is, and which gave the readings 
