EVAPORATION. 275 



and appear to come from a point nearer overhead than 

 they really do. Consequently the object appears higher 

 than it really is, and as we judge of the distance of ob- 

 jects by their apparent size, nearer. A similar appear- 

 ance is sometimes witnessed, though perhaps not to so 

 great an extent on land, when the air is full of vapor. 

 Hence it is a common remark in some places, ' We shall 

 have rain soon, for the hills look near.' The different 

 situation of hills, &c, makes this appearance more noticed 

 in some places than in others. It is well known that 

 fog when it is dense, almost wholly obstructs vision. It 

 has been asked why this should be, as fog is composed of 

 water, which is transparent. Those, however, who have 

 observed the appearance of an object, when viewed 

 through a globular vessel of water, can easily imagine 

 the effect produced by millions of exceedingly small glo- 

 bules, such as constitute fog. The principle of Optics, 

 on which this is explained, is mentioned above, but it is 

 necessary to mention that if the surface of the denser 

 medium be convex, the rays of light, in beaming nearer 

 perpendicular to that surface, are bent towards each other. 

 This may be seen in a common burning glass. If we 

 should attempt to look at an object through a pile of ten 

 thousand little balls of glass, we probably should not be 

 much better able to see it, than we are to see objects 

 through fog. 



Those who live near the Cape of Good Hope often 

 witness a striking phenomenon illustrative of our present 

 subject, when the wind blows from the southeast. ' Be- 

 yond the city, as viewed from the bay. there is a mountain of 

 great elevation, called, from its extended flat summit, the 

 Table Mountain. In general its rugged steeps, are seen 

 rising in a clear sky ; but when the southeast wind 

 blows, the whole summit becomes enveloped in a cloud of 

 singular density and beauty. The inhabitants call the 

 phenomenon the spreading of the table-cloth. The 

 cloud does not appear to be at rest on the hill, but to be 

 constantly rolling onward from the southeast ; yet to the 

 surprise of the beholder, it never descends, for the snowy 

 wreaths, seen falling over the precipice towards the town 

 below, vanish completely before they reach it, while oth- 



