464 ON THE SOURCES OF HEAT WHICH INFLUENCE CLIMATE. 
well accords with the remote era when the earth was yet uninhabited, and 
covered only with a thin stratum. It helps to explain the great revolutions to 
which those periods were subject; but since the historic era, the crust must either 
have acquired sufficient strength to overcome the force of these internal tides, or 
we may suppose the matter to have viscidity enough to resist the diurnal attrac¬ 
tive influence of the sun and moon. In every view there appear to me strong 
reasons for believing that the volcanoes have their origin at no great depth, and 
are entirely due to local causes. Further, admitting the existence of a primitive 
central heat, its power to retain the central mass of our earth in a fluid state 
appears to me very problematical. 
A distinguished French philosopher, M. Fourier, has made the influence of a 
central heat on the temperature of our climates the subject of rigorous mathema¬ 
tical investigations. He has shown its existence to be perfectly consistent with 
a uniform temperature of climate since the commencement of the historic era. 
The planetary mass will require a period of 4,000 years to lose by spotaneous 
cooling one-sixth of a degree of Fahrenheit, while the loss of heat through the 
cooling of a century is much less than the loss of solar heat as we pass from season 
to season. 
Fourier has further shewn, that the influence of the primitive heat in our 
planet is scarcely felt by the climates on its surface. Their temperature cannot 
on an average be raised above one-sixth of a degree of Fahrenheit through this 
source; so that local improvements may elevate the temperatures far more than 
they can ever be deteriorated by any loss of primitive heat, if indeed that source 
be at all capable of ever sensibly affecting them. These results of Fourier are 
of great value, since they restore confidence where the researches of Buffon and 
Leslie made the permanent stability of our climate doubtful. History is also 
strongly in favour of a fixed standard for our climates ; where there is a change, 
it is an improvement; for we know the climate of modern Greece and Italy to 
have much milder winters than those assigned them by the ancient Greeks and 
Homans. 
Jefferson was of opinion that the climate of the United States was rapidly 
improving; but more recent observers would seem to say, that the clearing 
of that country has the effect of elevating the summer heat, while it equally 
depresses the cold of winter. 
The effect of the temperature of the regions of space on the climates of the 
globe is a question that originated entirely with M. Fourier ; for what we know 
of the subject we are indebted to him. He has assigned the temperature of 
minus 58, Fahrenheit, or colder than freezing mercury, as the heat of the planet¬ 
ary space. Of its influence on our climates it will be best to quote his own 
words, as given in a memoir in the Annales de Chimie et Physique. He says 
