50 M. POUILLET ON SOLAR HEAT, 
Dates of the Series. Values of p. Values of 1 — p. 
DRE 2S Ok oa ts he eae 0:2756 
July 27 nips jataoog igen ba OF sane 0°2415 
September 22 . . . 0°7780 0°2220 
May eo. te agar 0:2444 
May ily ve ie. (OWES 0°2112 
Winter solstice . . . 0°7488 0°2512 
By means of these values of A and of p, and of the formula 
NS iat 
I calculated the results contained in the fourth column of the 
preceding table; it is seen with what accuracy all the num- 
bers which had been given by observation are thus reproduced, 
even when the observation corresponds to atmospheric thick- 
nesses, which are quadrupled by the effect of obliquity. Thus 
in the experiments of the 4th of May, the solar rays had to 
traverse an atmospheric thickness of 24 leagues at noon, and 86 
leagues at six o’clock in the evening, and yet the number cal- 
culated is still found perfectly to agree with the number ob- 
served. It is understood, however, that it is only when the 
weather is quite settled that the formula can be applied with 
accuracy to an entire day with the same value of p; if there 
occur any sudden changes in the state of the atmosphere, the 
values of p immediately experience a greater or less alteration : 
I have succeeded in convincing myself of this by a multitude of 
experiments corresponding to all the seasons of the year. It 
may even be assumed that in certain spots, especially in moun- 
tainous countries and near the sea-coast, the values of p undergo 
every day periodical variations, corresponding to the diffusion 
and to the condensation of the vapours. . 
5. If in the preceding formula we suppose p = 1, or == 0, we 
find t= 6°72; 
that is to say, that the pyrheliometer would assume an elevation 
of 6°-72 if the atmosphere could transmit wholly all the solar heat 
without absorbing any of it, or if the apparatus could be trans- 
ported to the limits of the atmosphere to receive there without 
any loss all the heat which comes to us from the sun, This 
value of ¢ multiplied by 0°2624, gives 5; 
1°7633. 
This then is the quantity of heat which the sun gives in 1! on 
a square centimetre, at the limits of the atmosphere, and which 
