and Moon on the Atmofphere, ^61 



In the atmofphere we have at prefent no proof of diurnal 

 tides, which ought to be the nioft apparent *. 



The elevations and depreffions of the barometer, "which ap- 

 pear to he periodical^ are fometimes more, fometimes lefs, con- 

 fiderable ; but in moft cafes, when they are regular, greatly 

 exceed the proportion which they ought to bear to the ex- 

 tremes of the fcale, on the fuppofition of their being due to 

 the relative pofitions of the fun and moon only. They arrive 

 at their extent fometimes before, fometimes after, the time of 

 the moon's phafes, and their direcliion is even fometimes 

 contrary to the theory propofed, for many days together ; in 

 all which particulars they difagree with the tides of the ocean. 



Now in order to a proper confideration of thefe differences, 

 it is neceflkry to keep in view the different conftitution of 

 the two fluids, which are the fubjefts of the comparifon. 



The ocean is a denfe fluid, incumbent on the folid earth, 

 pretty much alike in temperature and compofilion through- 

 out, fubjcft indeed to certain currents which are found to be 

 pretty conftant and appreciable. It does not appear to vary 

 in quantity. It has a well-defined furface, by which we can 

 meafure the alterations of level it is fubjeft to : 



The atmofphere is a much rarer and an elaftic fluid, in- 

 cumbent partly on trafts of land, of various qualities, partly 

 on the moveable furface of feas, partly on fnow and ice. It 

 differs, at different times, in quantity, and varies much in 

 denfity, temperature, and compofition; is moveable through 

 its whole extent by different and oppofite currents, for the 

 moft; part uncertain and changeable ; and we know nothing 

 about its furface. Thefe circumftances being confidered, we 

 (liall not expeft, even in theory, the fame regularity in the 

 tides of the atmofphere as in thofe of the ocean. 



The want of fafts to prove the exiftence of diurnal tides 

 appears indeed at firfl view an infuperable difficulty; fince, if 

 thefe did take place, the barometer ought to indicate them, 



• The means of afcert-iining thefe were not fo obvious in the time of 

 Newton ; and it is accordinyly faid, in B. II. chap. 6, of Pemberton's 

 Newton, that " the gravit.uion of thefe fluids (water and air) in the 

 earth towards tiie moon produces no fenfible efie£t, except in the fea, 

 where it caufes the tides. " 



Vol. VII. 3 A by 



