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 

 Romans. 



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 Annates de Chimie et Physique. He says : — 



