Temperature of the Globe. 335 



mountains, by thermoscopic apparatus sunk into the sea, it has 

 been possible to determine the velocity of the cooling process 

 which takes place at different seasons, from below upwards, in the 

 atmosphere, and from above downwards, in the occean, and in 

 fresh water lakes. The animals, therefore, which dwell in both 

 these elements, find on each point of the globe, in the aenfomi 

 and liquid elements, the most heterogeneous climates, placed in 

 strata one above another. In the depth of the sea, under the Line, 

 and in alpine lakes of the temperate zone, there is always a fixed 

 degree of cold, viz. that degree at which the water attains the 

 greatest density. The experiments of Ellis, Forster and Saus- 

 sure, have been repeated under all zones and in all depths ; but 

 what we know of the lowest temperature of the air, and of sea- 

 water, as well as of the greatest effect of the radiation of heat be- 

 tween the tropics, serves 'as an infallible proof that the cold 

 which there exists near the bottom of the sea, is produced by a 

 current which, in the depths of the ocean, passes from the poles 

 towards the equator, and cools the inferior strata of water in the 

 southern ocean, like the current of air in the upper atmosphere, 

 which moves from the equator to the poles, to temper the cold 

 of the winter in the northern regions. 



The immortal Benjamin Franklin first taught us that sand- 

 banks are sooner recognised by the thermometer than by the 

 sounding line. They are islands of the submarine land, which 

 the elastic subterranean potvers had not been able to elevate 

 above the surface of the water. On the declivity of the shoals, 

 the inferior and colder strata ascending by impulse, are mixed 

 with the upper and warmer ones ; and thus the sudden cold of 

 the sea-water shews to the navigator that danger is near. The 

 shallows, by their temperature, act on the air above thefin, in 

 which they produce fogs and groups of clouds, which are per- 

 ceived at a great distance. 



Before more extensive investigations had been made on the 

 distribution of heat over the globe, it was believed that the cli- 

 mate of two places could be determined by the extremes of the 

 temperature in summer and winter. This view of things has 

 still been preserved in popular opinion, whilst naturalists have 

 long ago renounced it as erroneous ; for, although undoubtedly 

 the extremes of single days and nights are in a certmn proper- 



