METEOROLOGY. 213 
(supposed to be near the point of saturation) be mixed, a precipitation of 
some of the water in the form of a fog will result. Pl. 24, fig. 7, is 
intended to illustrate the formation of fog at a certain place in England on 
a particular day. The temperature of the surface of the river was 56°, that 
of the air six feet above the water 473°, that of the land on the bank of the 
river 45°, and that of the air above the bank 49° F. The greater heat of 
the water, compared with that of the incumbent air, caused the formation 
of fog over the river; as, nowever, the temperature of the river bank was 
less than that of the air over it, the outlines of the fog were confined to the 
surface of the river. Sig. 8 shows the overtopping of the trees by the fog. 
Fig. 9 is an example of the formation of fog over a river inclosed by 
hills. 
Counting every day during which fogs are present for any length of 
time as a foggy day, we shall find that for one and the same place, the 
number of such days in different years is very nearly the same; not so, 
however, in different places. It has been observed that by far the greatest 
number of fogs experienced in London, Hamburg, Berlin, Stuttgard, and 
Munich, occurred in winter and autumn, while in Moscow foggy days are 
as numerous in summer as in winter. 
Fogs are much rarer on level lands than in mountainous regions ; on the 
dry plains of Asia and Africa they are almost entirely wanting. Next to 
mountains they are thickest on the shores of large bodies of water, and are 
more abundant as we approach the poles from the equator. They are 
especially prevalent in the Northern Atlantic in the neighborhood of New- 
foundland, New Scotland, and Hudson’s Bay, all noted for their fogs. The 
reason of their occurrence in these places is the condensation of the vapor 
arising from the Gulf Stream by the colder air. It often happens that fogs 
are very thick on the decks of vessels, while the tops look out on a clear 
sky. The coast of California is almost continually veiled in fogs ; even on 
the coast of Peru they last from four to five months at a time. Fogs are 
also frequent on the coast of Norway and of England. Beautiful exhibitions 
of fog occur on the English coast, particularly on the hills inclosing the 
harbor of Plymouth (see pl. 24, figs. 13-18). Indicating the hill to the left 
by A, the promontory in the centre with the tower by B, and the wooded 
hill by C, then in fig. 13 we see a strong condensation of vapor over A, 
observed about 5 P.M., of July 22, and-only a slight one over B. Half an 
hour later A presented the same appearance, but the fog had increased over 
B, and covered a part of C. About 6 P.M. the fog had increased over A 
and completely enveloped B; it had become shorter but higher over C. 
On the 2d of June, about 8} A.M., the fog had the appearance presented in 
Jig. 14, although the upper outline of the fog was not so regular as in our 
figure. About 4 P.M. the fog over A had entirely vanished, as in fig. 16, 
a dense cloud apparently rested on the hill, and seemed to hang over the 
water, together with the fog; B and C were, however, completely enveloped 
in fog. Fig. 15 represents a fog resting on the water against C, having a 
regular outline above, and concealing half the mountain. 
Isolated masses of fog are often observed on mountains, having only a 
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