ON THE EFFECTIVE TEMPERATURE OF THE SUN. 
387 
to turn the whole radio-micrometer through an angle of 180°, so that the heat from 
the platinum should now fall on that side of the receiving surfaces on which previously 
the sun had shone. The different positions are distinguished as follows :— 
Position (lA) = platinum heating upper circuit, and behind the small mirror fixed 
to the fibre of the radio-micrometer. 
(iB) = platinum heating lower circuit, and again behind mirror. 
,, (2A) = instrument rotated through 180°; platinum in upper circuit, in front 
of mirror. 
,, (2B) = platinum in lower circuit, again in front of mirror. 
The difference between positions lA and 2A, and between IB and 2B, w^e should, 
d priori, expect to be small, and the experiments show that this is so, while, as we 
have already mentioned, the larger differences between the A and B positions were 
also to be anticipated from unavoidable dissimilarities in the two parts of the 
combined circuit. 
One further point remains to be noticed, viz., that the geometrical mean of the 
mean temperatures of the A and B positions is not exactly the mean tempera,ture to 
be deduced from the observations, on account of the curvature of the radiation curve. 
To show what difference exists between the geometrical and the true mean, we may 
take the following numerical example : 
Mean balancing temperature in position A = 1600° absolute 
> 5 
B = 1300° 
Mean balancing temperature = 1443° 
Now, to a temperature of 1600°, corresponds a radiation of 312 in our arbitrary units ; 
to a temperature of 1300°, a radiation of 136 ; mean radiation = ^ (312 + 136) = 224. 
But to a radiation of 224, corresponds a temperature of 1472°, which is 29° higher 
than the geometrical mean, and the value of /\/( — \ = 4'5 approxi- 
mately. That is to say, we must add 29x4’5 = 130 to the mean temperature. A 
correction of about 100° is therefore to be made on the final mean of all the observa¬ 
tions, the separate details of which now follow. Each day’s results are given by 
themselves, with data sufficiently full to allow of any single observation l^eing 
calculated out. 
The date, height of barometer, and notes on the wmather are given first; hygro- 
metrical readings are not given, as no useful deductions can be made from them, as 
Rosetti points out in his paper. 
In the 1st column, the position is noted. 
,, 2nd ,, the local time of the observation. 
3 D 2 
