THE EARTH. 



109 



exposes a greater surface to the vaporating 

 rays. A dry frost, in some measure, assists 

 the quantity of evaporation ; as the quantity 

 of rtys are found to be no way diminished 

 th<> 'by. Moist weather alone prevents eva- 

 poration; for the rays being absorbed, re- 

 fracted, and broken, by the intervening mois- 

 ture, before they arrive at the surface, cannot 

 produce the effect; and the vapour will rise 

 in a small proportion. 



Thus far we have accounted for the ascent 

 of vapours; but to accountfor their fallingagain 

 is attetided with rather more difficulty. We 

 have already observed, that the particles of 

 vapour, disengaged from the surface iof the 

 water, will be broken and attenuated in their 

 ascent, by the reflected, and even the direct 

 rays, that happen to strike upon their minute 

 surfaces. They will, therefore, continue to 

 ascend, till they rise above the 'operation of 

 the reflected rays, which reaches but to a 

 certain height above the surface of the earth. 

 Being arrived at tin's region, which is cold 

 for want of reflected heat, they will be con- 

 densed, and suspended in the form of clouds. 

 Some vapours, that ascend to great heights, 

 will be frozen into snow : others, that are con- 

 densed lower down, will put on the appear- 

 ance of a mist, which we find the clouds to 

 be when we ascend among them, as they hang 

 along the sides of a mountain. These clouds 

 of snow and rain, being blown about by winds, 

 are either entirely scattered and dispersed 

 above, or they are still more condensed by ; 

 motion, like a snow-ball, that grows more 

 large and solid as it continues to roll. At last, 

 therefore, they will become too weighty for 

 the air which first raised them to sustain ; and 

 they will descend with their excess of weight, 

 either in snow or rain. But, as they will fall 

 precipitately, when they begin to descend,the 

 air, in some measure, will resist the falling: 

 for, as the descending fluid gathers velocity 

 in its precipitation, the air will increase its 

 resistance to it, and the water will, therefore, 

 be thus broken into rain ; as we see that wa- 

 ter which falls from the tops of houses, though 

 it begins in a spout, separates into drops 

 before it has got to the bottom. Were it not 

 for this happy interposition of the air, be- 

 tween us and the water falling from a consi- 

 derable height above us, a drop of rain might 



fall with dangerous force, and a hailstone 

 might strike us with fatal rapidity. 



In this mariner, evaporation is produced 

 by day ; hut when the sun goes down, a part 

 of that vapour which his rays had excited, 

 being no longer broken, and attenu ted by 

 the reflecting rays, it will become heavier 

 than the air, even before it has reached the 

 clouds; and it will, therefore, fall back in 

 dews, which differ only from rain in descend- 

 ing before they have had time to condense 

 into a visible form. 



Hail, the Cartesians say, is a frozen cloud, 

 half melted, and frozen again in its descent. 

 A hoar frost is but a frozen dew. Lightning 

 we know to be an electrical flash, produced 

 by the opposition of two clouds : and thun- 

 der to be the sound proceeding from the 

 same, continued by an echo reverberated 

 among them. It would be to very little pur- 

 pose to attempt explaining exactly how 

 these wonders are effected ; we have as yet 

 but little insight into the manner in which 

 these meteors are found to operate upon each 

 other; and, therefore, we must be contented 

 with a detail rather of their effects than their 

 causes. 



In our own gentle climate, where nature 

 wears the mildest and kindest aspect, every 

 meteor seems to befriend us. With us, rains 

 fall in refreshing showers, to enliven our fields, 

 and to paint the landscape with a more vivid 

 beauty. Snows cover the earth, to preserve 

 its tender vegetables from the inclemency of 

 the departing winter. The dews descend 

 with such an imperceptible fall as no way in- 

 jures the constitution. Even thunder is sel- 

 dom injurious ; and it is often wished lor by 

 the husbandman, to clear the air, and to kill 

 numberless insects that are noxious to vege- 

 tation. Hail is the most injurious meteor 

 that is known in our climate ; but it seldom 

 visits us with violence, and then its fury is 

 but transient. 



One of the most dreadful storms we hear 

 of," was that of Hertfordshire, in the year 

 1697. It began by thunder and lightning, 

 which continued for some hours, when sud- 

 denly a black cloud came forward, against 

 the wind, and marked its passage with de- 



Phil. Trans, vol. ii. p. 147. 



2A 



