76 THEORY of RAIN. 



come naturally to be mixed on thofe occafions of inteftine mo- 

 tion, we fhould have, according to the theory, an immediate 

 caufe fomewhere, in all probability, for the condenfation of 

 humidity, or production of rain. It does not, however, ap- 

 pear why the falling barometer mould indicate rain any more 

 than the rife of the mercury ; at the fame time, it may be 

 made to appear how, in general, the gradual rife of the ba- 

 rometer, to its greateft ordinary height, and its continuation in 

 this growing ftate, mould naturally indicate fair weather, or 

 much fair weather, in the quarter of the globe concerned with 

 thofe obfervations. 



IN order to perceive this, let us fuppofe fuch fair weather to 

 take place, and that there is an undifturbed atmofphere in this 

 quarter of the earth ; then it will appear, that the neceffary eva- 

 poration from the furface of the globe muft gradually increafe the 

 weight of the atmofphere, or its height, or both. But, in this 

 cafe, the mercury in the barometer muft rife with a gradual 

 progrefs, while there is no motion in this quarter of the atmo- 

 fphere, fufficient to caufe a general rain, or to make a change in 

 this natural ftate of the barometer. Thus, though, perhaps, 

 there is not any condition of the barometer that pofitively indi- 

 cates rain, we have, from the obfervations of that inftrument, a 

 pofitive indication of fair weather. 



BUT, though this indication of fair weather be pofitive in 

 its nature, as being founded on principle, and not on fuppofi- 

 tion; yet it is only true conditionally; that is, providing no 

 other caufe interferes, or that the ftationary place of the mer- 

 cury be not occafioned by the contrary operation of different 

 changes. This deceitful occurrence, however, though often 

 happening in fmall degrees, will rarely prevail to fuch an ex- 

 tent as might render doubtful, in general, the pofitive indica- 

 tion of the barometer, in relation to fair weather. 



HAVING thus obtained a pofitive indication of what may be cal- 

 led the fair-weather feafbns, and the raiay feafons, requiring com- 

 motions 



