RADIATION AND ABSORPTION. 47 
In my apparatus this quantity of heat received in one minute 
by each square centimetre is 
0°2624 ¢. 
The pyrheliometer with a lens consists of a lens of 24 to 25 
centimetres diameter, having a focal distance of 60 to 70 centi- 
metres, in whose focus is placed a silver or plated vessel contain- 
ing about 600 grammes of water; the form of the vessel and the 
arrangement of the lens are so combined that for all heights of 
the sun the rays fall perpendicularly on the lens and on the 
face of the vessel which is to receive them at the focus and to 
absorb them. 
The experiments are performed as with the preceding appa- 
ratus, and the quantities of heat which fall in one minute on 
each square centimetre are determined by an analogous formula ; 
only there is an additional correction to be made for the quan- 
tity of heat which the lens absorbs, and that correction is made 
by the comparison of the results obtained with the lens and with 
the direct apparatus. Among the lenses which I have tried, 
that which absorbed the least nevertheless absorbed one-eighth 
of the incident heat. 
It is necessary to employ the pyrheliometer with a lens when 
the experiments cannot be made in a calm air; the wind, when 
it is not strong, has but small influence in cooling within five 
minutes a mass of water of more than 600 grammes, which is 
raised only four or five degrees above the surrounding tempera- 
ture, so that the correction is always very small. 
2. The following table contains five series of experiments, 
which will give a sufficient idea of the variations of the direct 
pytheliometer. The elevations of temperature observed are in 
the third column. We shall show hereafter how the numbers 
of the second and fourth columns have been obtained. 
