242 Prof. Hennessy on the Distribution 



distribution of heat over the earth. This elegant method of re- 

 presentation has since been appUed to other elements of terres- 

 trial physics, such as atmosphei'ic pressure, magnetic intensity 

 and declination^ and the distribution of the tides. 



While the science of terrestrial temperature was thus pro- 

 gressing by systematic induction from observed facts, some of its 

 fundamental principles were examined and consolidated by the 

 mathematical labours of Fourier, Poissou, and Laplace. Fourier 

 especially has established with surpassing clearness the relations 

 subsisting between the thermal conditions of the earth's surface, 

 the warmth of its interior, and the temperature of the planetary 

 spaces. His works also contain some highly suggestive views 

 as to the influence of the physical properties of the superficial 

 portions of the earth's crust on local and general climates. 



No one has more ably extended, or more effectively applied, 

 Humboldt's graphical method of representing the distribution of 

 terrestrial temperature than Professor Dove. Having tabulated 

 a vast number of observations made in different parts of the 

 world, he has not only been able to improve the representation 

 of mean annual temperature, but has furnished us with maps of 

 the isothermals of each month*. 



A glance over the maps of mean annual, mean summer, mean 

 mnter, or mean monthly temperature, informs us of the general 

 fact, that the temperature of a place depends on other circum- 

 stances besides its latitude. Mayer's law could not be true unless 

 the isothermals were all parallel to the equator, — a relation which 

 they are far from fulfilling. The same remark applies to the 

 improved forms of Mayer's law, which have been subsequently 

 suggested by different eminent scientific men f- Had the earth's 

 surface been of uniform texture, and stripped of every kind of 

 fluid covering, the temperature of every point would depend upon 

 its latitude, and places having the same latitude would possess 

 the same temperature. But three-fourths of the heat-absorbing 

 and heat-radiating surface is water ; and this, from its mobility 



* These maps appeared originally in the Berlin Transactions, but they 

 are best knomi in these countries through the English edition prepared for 

 the members of the British Association by General Sabine. 



t Except a formula of Sir David Brewster's, in which he makes the 

 mean temperature of a place depend upon its distance from two points of 

 minimum temperature, the one in Asia, the other in America. This was 

 undoubtedly an improvement; but it did not justify the remark of an emi- 

 nent writer, that the coordinates of latitude and longitude should be alto- 

 gether discarded in connexion with climate. The connexion of the former 

 is obvious, though complicated : the connexion of the latter is less import- 

 ant, and it manifestly depends on the prevalent directions of oceanic and 

 aerial currents, the influence of which on climate is luiiversally acknow- 

 ledged. See Forbes, Rejiort of the British Association, vol. i. ]). 215, 

 second edition. 



