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Show .. f 186 HOAR FROST. able on a dty frosty day, a day even of the keenest frost, than on those raw days which are not exactly either frost or thaw, or even when thaw comes slowly, and the surface is covered with melting ~now and ice. The hard ice and unmelting snow 1n the clear frosty day affect the air very little, whereas on the raw day, and when it thaws partially, the air is loaded with moisture, which takes the heat out of it: and as, on such days, there is seldom any direct sunshine to assist in dispersing the moisture up into the air, it hangs in the lower strata like a heavy fog, and abstracts heat from the human body, and forms hoar fro::;t upon the hair and clothes : and whenever the temperature sinks a little, the water is deposited and crystallized upon every solid substance, and the more so the more slender the substance is, so that the grass and bushes and the twigs of the trees are frosted over with spiculre of ice, · which have a very pretty but very cold appearance. Those hoar frosts are most frequent in the autumn, before the waters be so far cooled down as that ice or dry frost is found. They sometimes occur late in the spring and in cold districts occasionally even in the summer. If in the latter part of the season those hoar frosts, or white frosts, "hold," by a continuance of the col<l atmosphere near the surface, they generally end in dry or black frost, and are followed by cold, but healthy and hearty winter weather. But if that air near the surface be warmed by any cause, so that the frost "gives way," or, as it is called in some parts of the country, "leaps," then, if the cause of that be general, rain is the immediate consequence, even though the general progress of the season be such as ultimately to lead to black frost, and a heavy fall of snow. It may seem a little contradictory that temporary local heat should produce cold, but it is nevertheless true, in that as well as in other cases. How soon CATCHING COLD. 187 a person who has been in too close a room, or too n~ar the fire, gets cold. and s~ivering, compared With one ~ho has been m a col<1cr apartment at a gr~ater distance from the fire,_ or in. the oped air. Half the ~olds an~ coughs WJ.t? whwh people are ann~ye_d m t~e wmter are owmg to their winter habitatiOns bemg too w~rm: and those complaints are far more frequent m towns than in the open places of ~he country. When people go hot into the .. cold ~Ir, ~he evaporation from the surfaces of their bodies Is_ so rapi_d,_ as not only to make them feel~old and s_htver, but 1f 1t be long continued, to injure the httle follicles of the skin, which, in the heaithy states of_ the body, remove much of the waste matter that IS unfit for the purposes of life · and thus th~t matter re~ains. in the system, and' acts as a poison. Washmg with warm water in cold weather has ~uch the same. effect ; and they who resort to that m order to a_void the temporary influence of the cold, thereby subJect themselves to it for the whole day. In summer, warm. water. is a luxury, and a wholesome, and almost Immediately a cooling luxury: ~ut they who would escape chilblains and frost-biting should avoid it in winter. The temporary warmth of the air, which melts th~ hoar fr?st, acts in a similar manner. As the sp1culre of IC~ thaw (and very little heat thaws them as they are Ill small needles to which the air ha~ access on all sides), the water evaporates, and soon takes as much heat from the atmosphere as cools that more than ever; a~d the cooling influence gradually extends up~ard, till all the vapour in the sky up to a very considerable height, is much colder tha~ befo:re. As the heat diminishes, the tendency of the particles of water to each · other which has been suspended, bu~ not in the least dest~oyed by the heat, returns to actwn, and the particles approaeh each other, and form a cloud. That cloud gathers vapour from all the space surrounding it, not only while it |