384 
MESSRS. W. E. WILSON AND P. L. GRAY 
The difference then between Rosetti’s and Langley’s figures is in a direction 
which might be expected, and the results deduced from the work of the former may 
be assumed provisionally as an approximation to the truth. 
Climatic conditions in Ireland are such as to entirely prevent a good series of 
observations on tliis point ; a perfectly clear sky from morning to night, with a fairly 
constant hygrometic state of the atmosphere, is extremely rare. 
Rosetti, working under the unchuided skies of Northern Italy, was able to make 
a large number of observations at all hours of the day, with very consistent and 
apparently reliable results. 
We have, therefore, determined to use the correcting factors for atmospheric 
absorption which have been deduced from his figures, so that whatever doubt may be 
thrown on the accuracy of his final result will affect ours in a certain proportion. 
It is worth noting that Young* gives 30 per cent, as the absorption in the zenith, 
but without indicating the means by which he arrives at this figure. 
The Solae Radiation. 
The general method of making the final experiments has already been described. 
The necessity for making observations with the sun shining (l) on the upper circuit 
of the radio-micrometer, (2) on the lower circuit, arises from the unavoidable differ¬ 
ence in the constants of the two circuits. No special care had been taken in the 
construction of the instrument to make the receiving surfaces of equal size, and even 
if this had been possible, the electrical constants must have differed somewhat. The 
only way of correcting for these differences is to take independent observations in the 
manner indicated, and to take the mean of the results. 
A considerable difference between the figures obtained in the two positions was 
to be anticipated, and it will be seen that experiment confirms the anticipation. 
As we have already pointed out, when a balancing temperature has been obtained, 
the ratio of the radiation from the sun to that from the platinum is obtained by 
multiplying together four factors. They are : 
(1) The ratio of the apparent area of the sun to that of the platinum, as seen from 
the receiving surface of the radio-micrometer. The former is obtained from the value 
of the sun’s semi-diameter, as given by the ‘ Nautical Almanac’ for the day of the 
observation. The latter is a constant, the same “ stop” being always used in every 
position. The angle subtended by a diameter of the stop was 4'^‘702;t if 
(T = angular diameter of the sun at the time of observation, we therefore have :— 
area of platinum /4-702Y 
area of sun \ a J 
^ “ The Sun,” ‘ Interiiafc. Sci. Series,’ p. 26'2. 
t A new stop was used after Sept. 8th; see p. 391. 
