Page:Willich, A. F. M. - The Domestic Encyclopædia (Vol. 1, 1802).djvu/205

181&#93; BAR is formed by the union of fire and water, or the solution of the latter in the former, it is impossible that the vapour can be condensed, un- til this union or solution be effected. The beginning of the condensation of the vapour, then, or the first symptoms of approaching rain, must be the separation of the fire which lies hidden or involved in the vapour. This may at first be slow and partial, or it may be sud- den and violent: in the former case, the rain may come on slowly, and after considerable intervals ; in the latter, it will be quick, and in a large quantity. With regard to the effect of this separation be- tween the fire and water, we shall only observe, that as it is gradual and slow, the barometer may in- dicate rain for a considerable time before it appears : or, if die sensi- ble heat communicated from the vapour to the atmosphere, should be absorbed by the colder parts, or be carried off by any unknown means, or prevented from affecting the specific gravity of the air, the ba- rometer will undergo no change ; and yet the vapour, being deprived of the heat necessary to sustain it, must descend in rain ; and thus it happens, that the indications of the barometer do not always hod true, respecting the changes of the wea- ther. Hence also it appears, that, though the specific gravity of die air is diminished, unless that dimi- nution proceed from a discharge of the talent heat contained in the va- pours, no rain will follow ; and thus die sinking of the barome- ter may prognosticate wind as well as rain, or sometimes no change whatever. Such is die ingenious theory of Dr. Black, who certainly has produced the most philosophic ex- BAR [1S1 plana tion yet offered on tliis appa- rently intricate subject 5 and though his reasoning may not enable us to solve all the difficulties occurring in meteorological investigations, we do not hesitate to say, that it has placed the nature and causes of these phenomena in the clearest point of view, as far as it is possi- ble to account for diem, in the pre- sent state of physical science. We shall, therefore, conclude with ob- serving, that though the wind is certainly not die sole cause of raising and depressing the mer- cury, yet, in our climate, it has a remarkable influence on die state of the barometer. For, if the mercury falls, when the wind blows from those quarters which generally are productive of much rain, such as the south and south- west winds are in die environs of London, there is no doubt but wet w r eather will speedily follow : on the contrary, if die quicksilver rises, while the wind blows from dry quarters, such as die northerly and easterly regions are to Bri- tain, then it is highly probable that the weather will become fair. But if the mercury rises during a soudi- erly wind, or falls while it blows from the north ; in both cases the prognostics arc extremely doubt- ful ; as it frequently happens, that the weather does not correspond to the temporary rising or falling of the quicksilver. A very curious new phenome- non deserves to be mentioned, con- cerning the state of this instru- ment, on particuiar oct asions. M. Schmidt, Professorot Mathematics at Pforte, in Germany, lately ob- served, that his barometer became luminous in the day time, while the horizon was covered With thunder-clouds ; and diat on the N 3 approach