qfthe Terrestrial Globe. 211 



of which would be misplaced here, go too slow in summer, and 

 too quick in winter. 



These details which we have been just perusing, would have 

 been wholly superfluous, had we not imposed upon ourselves 

 the rule to make no use, in this place, of the principles of me- 

 chanical reasoning, without having previously shewn how we 

 could experimentally establish its correctness. 



Whatever has hitherto been said concerning a flat wheel, may 

 evidently be applied, word for word, to any body, of any other 

 figure. 



For example, let us conceive to ourselves a sphere having a 

 rotatory motion upon itself, in virtue of some original impulse. 

 If its dimensions increase, its rate of rotation will diminish ; the 

 sphere will occupy a longer time in making a complete revolu- 

 tion. On the contrary, if the sphere contracts, its velocity will 

 increase ; it will require less time for each revolution. 



Our globe, then, what else is it than a sphere suspended in 

 space, and daily revolving upon its centre, in virtue of an origi- 

 nal impulse ? It follows, then, that if its bulk be augmented, it 

 ought, from day to day, to turn more slowly upon itself; and 

 if its size be diminished, its movement should be accelerated. 



The materials of which the earth is composed dilate by heat, 

 and contract by cold. Those who think that the earth is cool- 

 ing, by this very sentiment admit that its radius is diminishing 

 —that its volume is becoming smaller and smaller. But we 

 have just seen that the size cannot diminish without the rapidity 

 of rotation being increased. The question, then, whether the 

 earth, 2000 years ago, was at the same temperature that it is in 

 1834, resolves itself into this. Did the earth make a revolution 

 2000 years ago, in precisely the same time as it does now ? 



Under the former form, the question seems imperiously to re- 

 quire thermometrical results, of which the ancients had no idea. 

 But, in the latter, we shall find, in the observations they have 

 bequeathed us, the means of knowing if the duration of the 

 earth's rotation is still the same. 



What, in truth, is the duration of this rotation .? It is no- 

 thing else than a certain space of time, which astronomers used 

 in ancient times, as they still do in our own : it is, in a word, 

 what they called the sidereal day. That there may be no doubt 



