210 METEOROLOGY. 



Another important enquiry is — Does the earth possess a heat 

 proper to itself and independant of solar influence ? This question, 

 although apparently involving important considerations, is, so far as 

 it has relation to heat at the earth's surface, speedily answered. The 

 ideas of Descartes, Leibnitz, and Buflbn respecting the incandescent 

 state of the interior of the eaith, are by all recent investigations 

 rendered highly probable. But the conclusions which Buffon would 

 educe from this fact, are proved to be quite enoneous. According 

 to this writer, the heat disengaged from the earth is many times 

 greater than that which, even in summer, is received from the sun. 

 Fourier has, however, satisfactorily and even mathematically shown 

 that, although we allow that the earth at the depth of fifteen miles is in 

 a state of fusion through intense heat, the effect produced on the 

 surface is not more than one-thirtieth of a centigrade degree. This 

 cause, then, may be regarded as constant and inappreciable. 



From the many data which require to be taken into the account 

 when estimating meteorological calculations, it will be seen that it 

 must be one of the most complicated of all the physical sciences; it 

 is that in which it is necessary to spread our observations over the 

 greatest extent of the earth's surface, and to note the weather in the 

 greatest variety of local and geographical position. The accumula- 

 tion of facts from distant quarters, and a comparison of the affections 

 of the atmosphere, in the same moment, at different points, and at 

 the same point, during successive moments, may throw great light 

 on the comprehensive scheme which regulates the otherwise appa- 

 rently arbitrary and capricious fluctuations of temperature, humidity, 

 and atmospheric currents. By this means, within the last few years, 

 much infoimation has been gained respecting the laws which appear 

 to influence extraordinary stoims. These may be briefly stated :— It 

 has been observed, that tornadoes and hurricanes are whirlwinds, 

 ordinarily generated within the tropics, and possessing, in some cases, 

 a very limited diameter, say less than 100 miles ; that those fonned 

 north of the equator move to the north, in a curve which is parabolic. 

 For instance, storms which agitate the West India islands, proceed 

 in the first instance to the eastern shores of North America, then 

 bend north-easward towards Europe. Further, — that as these stonns 

 proceed in their course, their circumference becomes gradually widened, 

 so as in some cases to attain a diameter of 1,800 miles; it is also 



