Agricultiircd Meteorolor/y. 
323 
of a locality for the cfrowth of wheat by ascertaining only the 
mean tein))orature attained in the course of the day by the surface 
soil exposed to the sun's beams^ unless we knew how that tempe- 
borne out by observations which have come under his cognizance — espe- 
cially those respecting the King of Prussia's orange trees at IJerlin. It is 
plain, however, that it is a law which can only obtain within certain limits 
of variation (limits differing no doubt for every individual species of plant). 
Observations upon this subject would be invaluable, and the following 
experimental problem may be laid down for vegetable jihysiologists : — 
To determine within what range of variations from the menn the develo))- 
ment of vegetation is promoted for a given plant. There is a maxininm 
value of A, beyond whicb the plant would be injured or destroyed. The 
above corollary would at this point cease to be true (physiologically), and 
it may become a question if the Ibrmula of the sum of the squares does not 
then altogether cease to express the relations of temperature and vege- 
tation. 
On the hypothesis that a mean derived from certain variations is more 
promotive of vegetation than the same mean when resulting from greater 
uniformity in the range of limits, light may be thrown upon many ano- 
malous phenomena. Sunshine may have an indirect, as well as direct, 
ett'ect in exciting active vegetation, through the higher value of A which 
then necessarily results from the greater contrast of the diurnal and noc- 
turnal temperatures.^ 
No universally accurate laws on the relations between climate and the 
development of plants can be obtained until a far greater range and mass 
of facts have been accumulated. Quctelet proposes isanthesical lines 
{]\nes oi simultaneous floirering). We require also lines of equal variation 
between the maximum and minimum temperatures, to compare with the 
isothermal lines. Quctelet points out that every plant has its own nu- 
merical constant (or square of a certain number of degrees of heat), with- 
out which the phenomena of flowering, &c., cannot occur. For the lilac 
he fixed this at 42G4° Cent. A similar determination of this constant for 
every species of plant would be an important element in the data for 
Ibrming any exact and scientitlc agricultural theories. Such a table of 
Floral Constants might be for the enlightened agriculturist, in reference to 
some purposes, of scarcely less value than the table of Lunar Declinations, 
or that of Sines, &c., to the navigator. Its accurate determination con- 
nects itself with some of the most complex and subtle problems in phy- 
sical science, viz., the calculation of the quantities of heat, solar and 
atmospheric, received in a given time on a given spot. Pouillet has paid 
great attention to this subject (in his Mcmoire sur la Chaleur Solaire, 
&c.), uniting experiment with theory in his investigations, by means 
of his Pyrhehometer, which M. Gasparin seems to have chiefly used in 
pursuing his own observations upon this point. It does not appear what 
instruments beyond the common thermometer Quetelet used in his experi- 
ments on the lilac and other plants. 
To enter fully into explanations and applications of calorific formulae 
in a paper like the present would be impossible : we will only slightly 
allude to a neat equation of Pouillet's, in which the thennometrical eleva- 
tion caused by the direct solar action during a given number of minutes, 
is made a function of e, the atmospheric thickness traversed bj' the solar ray. 
That e is itself a variable dependmg onz, the suns zenith distance for the 
given place and instant of time, is obvious. If / be the observed ther- 
mometrical elevation, reckoned in degrees Centig., Pouillet finds: — 
t = a p' (c entering as an exponent on the second side), 
a is a constant expressing the unvarying calorific power of the sun, elimi- 
