THE KARTH. 253 



•lireet 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 this 

 region, which is cold for want of reflected heat, they wUl 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 condensed lower dov\Ti, will put on the appearance 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 con- 

 densed by motion, like a snow-ball, that grows more large and 

 solid as it continues to roll. At last, therefore, they will be- 

 come too weighty for the air which first raised them to sustain ; 

 and they wiU descend with their excesses of weight, either in 

 snow or rain. But as they %vill fall precipitately, when they 

 begin to descend, the air, in some measure, will resist the fall- 

 ing ; for as the descending fluid gathers velocity in its precipita- 

 tion, the air will increase its resistance to it, and the water will, 

 therefore, be thus broken into rain ; as we see, that water which 

 falls from the tops of houses, though it begins in a spout, separ- 

 ates into drops before it has got to the bottom. Were it not for 

 this happy interposition of the air, between us and the water 

 falling from a considerable height above us, a drop of rain might 

 fall with dangerous force, and a hailstone might strike us with 

 fatal rapidity. 



In this manner, evaporation is produced by day; but when 

 the sun goes down, a part of that vapour which his rays had ex- 

 cited, being no longer broken and attenuated by the reflecting 

 rays, it wiU become heavier than the air, even before it has 

 reached the clouds; and it will, therefore, faU back in dews,* 

 which diflfer only from rain in descending before they have had 



* It has been observed with surprise, that when a number of bodies are 

 exposed together to dew, some are quite wetted with it, while others re- 

 main dry. This circamstanco probably depends upon the goodness of the 

 body as a conductor of heat. Good conductors will part with their heat more 

 readily, and will therefore evaporate the dew again, whereas it will remain 

 upon bad conductors, which will not so easily part with their heat. If thi^ 

 explanation be the true one, it follows that bodies exposed to the dew and 

 dry, must have a lower temperature than those which remain moist. 



