CLASS IV. 2. 4. 1. OF ASSOCIATION. 429 



the earth, which is most distant from the centre, round which the 

 earth moves in her annual orbit, than on the side nearest that 

 centre. Whence it likewise follows, that the gravitation of bo- 

 clies towards the earth is greatest about six hours after noon, and 

 after midnight. 



Now when the sun and moon have their united gravitation oil 

 the same side of the earth, as at the new moon; or when the solar 

 attraction coincides with the greater centrifugal motion of that 

 side of the earth, which is furthest distant from the moon, as at 

 the full moon; and when this happens about noon or midnight, 

 the gravitation of terrene bodies towards the earth will be greater 

 about six hours after noon, and after midnight, than at any other 

 part of the lunar period; because the attraction of both these 

 luminaries is then exerted on those sides of the earth over which 

 they hang, which at other times of the month are more or less ex- 

 erted on other parts of it. 



Lastly, as heat and motion counteract the gravitation of the 

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

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

 out which life cannot exist, it seems to follow, that when our 

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

 life should be the least; and hence that those diseases, which 

 begin with torpor, should occur about six hours after the solar or 

 lunar noon, or about six hours after the solar or lunar midnight; 

 and this most frequently about six hours after or before the new 

 or full moon; and especially when these happen at noon or at 

 midnight; or lastly, according to the combination of these powers 

 in diminishing or increasing the earth's attraction to bodies on its 

 surface. 



The returns or exacerbations of many fevers, both irritative and 

 inflammatory, about six in the evening, and of the periodic cough 

 described in Sect. XXXVI. 3. 9. countenance this theory. Tables 

 might be made out to shew the combined powers of the sun and 

 moon in diminishing the gravitation of bodies on the earth's sur- 

 face, at every part of their diurnal, monthly, and annual periods; 

 and which might facilitate the elucidation of this subject. But I 

 am well aware of the difficulty of its application to diseases, and 

 hope these conjectures may induce others to publish more nume- 

 rous observations, and more conclusive reasonings. 



SPECIES. 



1 . Somni periodus. The periods of sleeping and of waking are 

 shortened or prolonged by so many other circumstances in animal 

 life, besides the minute difference between diurnal and nocturnal 



