ClassIV. 2.4. 1. OF ASSOCIATION. 429 



the earth, which is mod diftant from the centre, round which 

 the. earth moves in her annual orbit, than on the fide neareft 

 that centre. Whence it likewife follows, that the gravitation of 

 bodies towards the earth is greateft about fix hour-- after noon, 

 and after midnight. 



Now when the fun and moon have their united gravitation on 

 the fame fide of the earth, as at the new moon ; or when the 

 folar attraction coincides with the greater centrifugal motion of 

 that fide of the earth, which is furtheft diftant from the moon, 

 as at the full moon ; and when this happens about noon or mid- 

 night, the gravitation of terrene bodies towards the earth will be 

 greater about fix hours after noon, and after midnight, than at 

 any other part of the lunar period; becaufe the attraction of 

 both thefe luminaries is then exerted on tiiofe fides of the earth 

 over which they hang, which at other times of the month are 

 more or lefs exerted on other parts of it. 



Laftly, as heat and motion counteract the gravitation of the 

 particles of bodies to each other, and hence become either the 

 efficient caufes of vegetable and animal life, or the caufes with- 

 out which life cannot exift, it feems to follow, that when our 

 gravitation towards the earth's centre is greateft, the powers of 

 life fhould be the leaft ; and hence that thofe difeafes, which 

 begin with torpor, fhould occur about fix hours after the folar or 

 lunar noon, or about fix hours after the folar or lunar midnight ; 

 and this mod frequently about fix hours after or before the new or 

 full moon ; and efpecially when thefe happen at noon or at mid- 

 night ; or laftlv, according to the combination of thefe powers in 

 diminiihing or increafing the earth's attraction to bodies on its 

 fur face. 



The returns or exacerbations of many fevers, both irritative 

 and inflammatory, about fix in the evening, and of the periodic 

 cough defcribed in Seel. XXXVI. 3. 9. countenance this theo- 

 ry. Tables might be made out to fhew the combined powers 

 of the fun and moon in diminifhing the gravitation of bodies on 

 the earth's furface, at every part of their diurnal, monthly, and 

 annual periods ; and which might facilitate the elucidation of 

 this fubject. But I am well aware of the difficulty of its appli- 

 cation to difeafes, and hope thefe conjectures may induce oth- 

 ers to publifh more numerous obfervations, and more conclu- 

 five reafonings. 



SPECIES. 



1. Somni periodus. The periods of fleeping and of waking 

 are fhortened or prolonged by fo many other circumftances in 

 animal life, befides the minute difference between diurnal and 



nocturnal 



