ABOUT TERRESTRIAL TEMPERATURE. 79 
17. § L. The gradation of mean temperature from the Equator to the Pole empiri- 
cally represented. 
I have referred to the attempts which have been made during the last hundred 
years to find an empirical formula showing the gradation of temperature, from the 
Equator to the Pole, in the northern hemisphere. Setting wholly aside, and en- 
deavouring as far as possible to forget for the moment any formule of the kind, I 
started a fresh with the numbers given by M. Dovein his work already cited (par. 9), 
and, by a process considerably indirect, I obtained a formula which represents 
these numbers with greater exactness than perhaps any other yet employed. 
The form of the expression is indeed coincident with that probably first used by 
M. Kamrz, expressing and suggested by the physical fact, that the temperature of 
the globe (on an average of all meridians) reaches its maximum, not at the Equa- 
tor, but somewhat farther north ; according to this formula, in lat. 6° 30’ N. 
- 18. The formula (purely empirical) is this :— 
T=80%8 cos? (A—6° 30’), 
where T is the mean annual temperature on Fahrenheit’s scale, of the parallel 
whose latitude is A. 
19. The following table contains a comparison of this formula with numbers 
derived from M. Dove’s charts and tables :— 




Tasre I. 
en by Temp. Difference of ae By Temp. Difference of 
North Lat. ape oe by Rona: Formula. Mogi Eee ee ek by Formula.| Formula. 
0° 79°°7 79°:8 +0°1 60° 29°-8 28°°6 —1°2 
10 Wes 80-5 +0°6 65 22-6 22°] —0°5 
20 Miho 76 °4 —1:1 70 16 °4 I cal —0°:3 
30 69 8 G7 9 —1°9 75 10-7 10-8 +01 
40 56 °5 56 *2 —0°3 80 6-8 6°5 —0°3 
50 42:5 42 °5 0:0 90 2°3 1:0 = il os 





20. On this table I have to remark, that the numbers in the second column for 
the latitudes 50°, 60°, and 70°, differ by +0°-8, —0°4, and +0°-4, from those given 
by M. Dove at page 14 of his Distribution of Heat, &c. The numbers adopted 
have been deduced from a most careful investigation from M. Dovr’s more recent 
Polar Chart of Isothermal ‘Lines, in his Klimatologische Beitrdge, published in 
1857.* These numbers sensibly diminish a very manifest anomaly in the pro- 
* It is by no means an easy matter to deduce the mean temperature of a given parallel cor- 
rectly from an isothermal chart. What precise method M. Dove adopted I am not aware. After 
various attempts, I found the following procedure to be the only satisfactory one. Taking eighteen 
equidistant meridians (20° apart), I projected the temperatures indicated by the isothermal lines 
VOL. XXII. PART I. x 
