324 
Af/ricuUural Bletcorology. 
rature was to be calculated; and unless some account were taken 
of the constantly varying: ratio of the periods in which the heat 
accrues, and for which the author has provided no formula. He 
does not even advert to it. In the high latitudes it would seem 
that some other agent is at work to supply the requisite vegetative 
power, the extent of whose function has yet to be determined. 
Taking three places under different parallels whose mean 
hottest months are respectively 70°, 65\ 60° Fahr., which will 
nearly correspond to those of Toulouse, Berlin, and St. Peters- 
burg, we shall see that the total number of hours during which 
the solar heat accrues ought to be considered. 
At Toulouse, lat. 43° 36', the July temperature is 70° (21°T 
Centigrade); the day, on an average, is 14f hours long.* 
At Berlin, lat, 52^ 31', the July temperature is Go" (18° 3 
Centigrade), and the length of day is the same as in London, 
16T hours. 
At Petersburg, lat. 59^ 56', the July temperature is 60° 
Fahr. (17°T Centigrade), and the day is 18 hours long. 
If merely the number of days without reference to their length 
Avere calculated according to M. Gasparin's method, the results 
would come out thus : — 
Half of do. 
as due to 
Assumed the day 
Days. Solar lieat. only. Days. 
Toulouse . 2°l-l X 31 = C54-1 13 0-5 X 31 = 20°l-5 + 6o4°l = 855-6 
Berlin . . 18-3 X 31 = 5fi7-3 20 10 X 31 = 310 + 567-3 = 877-3 
Petersburg 17-1 X 31 = 53C'l 25 12-5 X 31 = 387 + 530-1 = 917-1 
But if the proportion of solar, not atmospheric, heat were com- 
nation made of all modifying forces, such as the absorptive influence of 
the atmosphere, &c. p is albo a constant, but is determined by conditions 
which are invariable for very limited periods only. It expresses the 
transmissive power of the atmosphere, and is obviously constant only for 
a given time and corresponding condition of that atmosphere. Pouillet 
calls a the solar constant, p tlie atmosplieric constant. He has obtained the 
above general formula from numerical equations lurnished by the tabulated 
results of observations carried on during some years. ]n these equa- 
tions t and e were the known quantities, the values of t being given by 
the pyrrheliometer, the values of e by astronomical and trigonometrical 
formulae, a &nA p were the two miltnon-n quantities to be determined. 
These reasonings and computations seem to contain the elements of 
correctness, and the equation proljably expresses the true physical rela- 
tions. He makes it the basis of many important deductions respecting 
the aggregate sums of heat received during certain periods — hours, days, 
months, or seasons — by the earth's surface ; computations which of course 
necessitate reductions foi- ohliquily of flie rays as well as other corrections. 
Any notice of these further conclusions, or discussion of their value and 
exactness, is however here inappropriate. A. A. L. 
* Snn rises at Toulouse, July 1, 41i. 24 m. ; sels 7 li. 30 m.1 , _ i j i jn 
31,4 47 7 13 f"''y - 
Petersburg,,, 1, 2 50 9 10 ), „ 
31, 3 40 S 20 / ~ " 
