14 M. Humboldt on Isothermal Lines^ and the 
form the term of comparison, is founded only in a small degree 
upon the observations of navigators, and to a great degree on 
the theory of Mayer. He has also confounded experiments 
made on the superficial temperature of the ocean with the re- 
sults of meteorological journals, or the indications of the tem- 
perature of the air which rests upon the sea : He has obvious- 
ly reasoned in a circle, when he modified, either by theoretical 
suppositions or by observations made on the air upon the coasts of 
continents, the table of the temperature of the ocean, in order 
to compare afterwards with these same results, partly hypothe- 
tical, those which observation alone furnished in the interior of 
the earth. After the works of Kirwan, we must notice those 
of Cotte, which are merely laborious, though useful, compilar- 
tions, which, however, ought not to be used without much cir- 
cumspection. A critical spirit has rarely presided over the re- 
duction of the observations, and thev are not arrang-ed so as to 
lead to general results. 
In detailing the actual state of our knowledge on the distri- 
bution of heat. I have shewn how dan^jerous it is to confound 
the results of observation with theoretical deductions. The 
heat of any point of the globe depends on the obliquity of the 
sun’s rays, and the continuance of their action, on the height of 
the place, on the internal heat and radiation of the earth in the 
middle of a medium of variable temperature ; and, in short, up- 
on all those causes which are themselves the effects of the rota- 
tion of the earth, and the inequal arrangement of continents 
and seas. Before laying the foundation of a system, we must 
group the facts, fix the numerical ratios, and, as I have already 
pointed out, submit the phenomena of heat, as Halley did those 
of terrestrial magnetism, to empirical laws. In following this 
method, I have first considered whether the method employed 
by meteorologists for deducing the mean temperature of the 
year, the month and the day, is subject to sensible errors. 
Assured of the accuracy of the numerical averages, I have 
traced upon a map the isothermal lines, analogous to the 
magnetic lines of dip and variation. I have considered them 
at the surface of the earth in a horizontal plane, and on the 
declivity of mountains in a vertical plane. I have examined 
2 
