SLOW COOLIXG OF THE EARTH. 113 



volcanic nature of which was known to the ancients 

 — such as Syria, the Greek Islands, and Southern 

 Italy — are to this day often visited by earthquakes 

 and outbursts. The activity of Vesuvius seems 

 not to have diminished since its first outbreak in 

 the time of the Emperor Titus. Many hot springs 

 which were used by the ancients, are still in 

 repute. 



A calculation made by La Place gives a very 

 remarkable proof of the extremely slow cooling of 

 the inner mass of the earth. From certain obser- 

 vations of the Alexandrian astronomer Hippau- 

 chus, the details of winch have come down to 

 us, the period of the daily revolution of the earth 

 on its axis — that is, the length of the sidereal day 

 at that time (150 B.C.) — may be determined with 

 accuracy. If, now, the mean temperature of the 

 earth's mass had diminished 1° Cent. (lf° Fain-.), 

 this must have contracted proportionally, and its 

 diameter and circumference must have become 

 less. If, then, the speed of the revolution of all 

 its parts remains unchanged, the time of its revo- 

 lution must have become shorter. If the mean 

 density of the earth were not greater than that of 

 glass, the sidereal day would, as La Place shows, 

 have become shortened by two seconds. But the 

 actual change winch has taken place since the 

 time of Hipparchus does not amount to the hun- 

 dredth part of a second* 



