216 M. Arago an the Thermometrkal State 



The change of temperature which we have just supposed in the 

 terrestrial radius, was therefore 170 times greater than it is pos- 

 sible to admit, according to the observations of the duration of 

 the sidereal day ; and hence, in 2000 years, the mean tempera- 

 ture of the general mass of the earth has not varied y ^ ^ of a de- 

 gree of the centigrade thermometer. 



We should add much, indeed, to the result of the uncertainty 

 which yet remains concerning the dilatability of the materials 

 which compose the globe, by multiplying it by 10, or, that we 

 may have an even number, by 17. This would extend it to 

 the 10th {^^) of a degree, beyond which the mean temperature 

 of the globe (the globe being always regarded in the mass, that 

 is to say, both the exterior and the interior being comprehend- 

 ed) has assuredly not varied in the space of 2000 years. 



V. Does the original heat of tJie globes still so apparent at a certain 

 depth, contribute, in a marked manner ^ to the actual temperature 

 of the surface? 



Marian, Buffon, and Bailly, so far as France is concerned, 

 estimated the heat which escaped from the interior of the earth 

 at 29 times in summer, and 400 times in winter, the quantity 

 that reached us from the sun. According to this view, the lu- 

 minary which enlightens us would contribute but a very small 

 share of that heat of which we feel the happy influence. 



This idea has been developed, with much eloquence, in the 

 Memoir es deV Academic^ in a paper upon the Natural Epochs 



will still be a simple second. In truth, it relates only either to the details gone 

 through at the point of departure, or to those at the point of arrival. Every 

 on^ then, can comprehend that these operations must be identically the same 

 at the two extremities of an arc, whatsoever be its length. When, to get 

 the diurnal arc, we divide by 10, the corresponding arc of 10 days, this division 

 will diminish the error in the whole arc in the ratio of 10 to 1 ; this error, 

 therefore, will not be more than the j'g of a second of a degree, corresponding 

 to t'j of a second of time. 



Finally, if we measure the lunar arc described in 200 days, when we come 

 to divide this total arc, which includes a great number of circumferences by 

 200, thus to procure the diurnal arc, corresponding to the medium of the in- 

 terval of 200 days, the only doubtful second by which this whole arc will be 

 affected, will become ia the diurnal arc a 200th of a second of a degree, corres- 

 ponding to a hundredth of a second of time. 



These explications must suffice in elucidation of the astonishing accuracy 

 assumed in the text. 



