128 POPULAR LECTURES AND ADDRESSES. 



seasons to the extent of a month and a half. But 

 observation proves that there has been a dislocation 

 of the seasons only to the extent of about an hour 

 and three-quarters, since a certain eclipse of the 

 moon was seen on March 19, 721 B.C., in Babylon. 

 It is quite certain, therefore, that meteoric supply 

 for sun heat has not within historical periods come 

 from distant space outside the earth's orbit. Me 

 therefore found it necessary to modify the meteoric 

 hypothesis of sun-heat a hypothesis which he 

 had learned from a communication by Mr. Watcr- 

 ston to the British Association at Hull in 1853, but 

 which he has since found had been previously 

 proposed by Mayer. If it is true that the heat 

 emitted by the sun is compensated from year to 

 year by meteors, he proved that instead of a certain 

 quantity of meteors falling in a certain time from 

 distant extra-planetary space, as supposed by 

 Mayer and Watcrston, a double quantity in the 

 same time must fall from orbits inside that of 

 Mercury. But at the same time he pointed out 

 that observation and dynamical theory of the 

 motions of the planets must be had recourse to, to 



