Chap, i j.] ibe Weather. 499 



. II. Among the various means of prognofticating 

 the weather, remarked by the late Mr. 'Adams f, 

 one of the moft important, in his opinion, feems to 

 be that undulating motion, or tumult in the air, which 

 is excited by the heat of the fun. The humidity 

 raifed from the earth by the heat of the fun, is fuf- 

 tained in the atmofphere by its heat, and the agita- 

 tion of the air. Though this motion is not always 

 vifible to the naked eye, yet by the help of a good 

 telefcope it becomes eminently confpicuous; every 

 object appears to be in violent agitation, and the 

 boundary line of the fenfible horizon, which would 

 otherwife be clear and well defined, is waved like a 

 field of corn agitated by the wind, or the furface of 

 the fea in a frefh gale. While thefe undulations con- 

 tinue in the air, the vapours remain there ; but when 

 the fun departs, and they fubfide, thefe aqueous par- 

 ticles become condenfed, and defcend to the ground 

 during the night, and in the morning affume the ap- 

 pearance of dew. 



III. The greateft arquifition, perhaps, that ever was 

 made to natural philofophy, with refpect to afcer- 

 taining the changes of the weather, was the difcovery 

 of the Barometer. The nature and ufes of this inftru- 

 ment have been previoufly defcribed J. It is evident, 

 - that when the mercury rifes in the tube, the preffure, 

 weight, or denfity of the air muft be augmented ; but 

 the relation that exifts between this preffure and the 

 change of weather, which does not take place feme- 

 times till ten or twelve hours afcerwards, ftill remains 

 to be explained. 



The preffure of the air upon the refervoir of the 

 barometer proceeds in general from its weight, and 



f Diflertation on the Barometer. 

 I See this Book, Chap. IX. 



K k 2 fometime* 



