CHAMBERS'S INFORMATION FOR THE PEOPLE. 



vapour or clouds must necessarily be frozen, and the 

 descending particles will be snow instead of rain. 

 Snow-flakes are the aggregation or union of frozen 

 particles, just as rain-drops are the union of watery 

 particles. They aggregate, according to the law 

 of the crystallisation of water, into regular and 

 symmetrical forms, of which the general character 

 is a six-sided figure ; as, for example, six needles 

 branching from a centre, or six arms from a six- 

 sided nucleus, each needle being three or six sided. 

 Though single crystals always unite at angles 

 of 30, 60, or 120, they nevertheless form, by their 

 different modes of union, about 1000 distinct 



varieties of snow-flake, some of which are figured 

 in the preceding engravings. Any agitation of the 

 air, or an increase of moisture or temperature, 

 destroys, of course, their delicate and beautiful 

 structure. 



Hard pieces of ice falling in showers are called 

 hail. The mode of their formation is not clearly 

 understood. The sudden ascent of moist air into 

 the upper regions, where it encounters a cold 

 current, is probably the common cause of hail. 

 Hailstones vary in shape, and when cut across are 

 found to be composed of alternate layers of clear 

 and opaque ice, enveloping a white nucleus. Many 

 of them seem to be agglomerations of several hail- 

 stones. They vary in size from the smallest shot 

 to several inches in diameter. 



Besides nitrogen, oxygen, and -watery vapour, 

 the atmosphere contains minute quantities of 

 numerous other substances carbonic acid, am- 

 monia, ozone, exhalations of all kinds, dust, seeds 

 or germs of plants and animalcules, &c. But, 

 however important some of them may be in the 

 economy of nature, the consideration of them 

 does not properly belong to meteorology. 



SURFACE OF THE EARTH IN RELATION TO 

 THE ATMOSPHERE. 



Mean Temperature Trade-winds Sea and Land Breezes 

 Hygrometric Changes Fall of Rain Fluctuations of the 

 Barometer and Thermometer. 



The changed and fluctuations of the atmosphere 

 have all a relation to the peculiarities of the earth's 



38 



surface, and especially to the unequal heating of 

 its different parts, owing to the varied action of 

 the sun, the distribution of sea and land, and the 

 different elevations of the land. 



By the mean temperature of a place is under- 

 stood the average temperature for a whole year, 

 or for a number of years. If observations were 

 made of the temperature for every hour in the 

 course of a day, the average of all these would be 

 the mean temperature of the day ; also the mean 

 of the greatest and least temperatures would be 

 pretty nearly the mean of the day. If the mean 

 temperatures of 365 days are found in this way,, 

 and an average taken of the whole, this gives the 

 average or mean temperature of the year. And if 

 a great many years have been observed in the 

 same manner, the average of the whole would be 

 reckoned the general mean temperature of the 

 place where the observations have been made. 



The action of the sun is the chief source of the 

 warmth possessed by the atmosphere and the 

 ground, though not the only source (see GEOLOGY 

 and PHYSICAL GEOGRAPHY). 



The reason why the sun heats the equator and 

 the regions adjoining more strongly than it heats 

 the temperate and polar regions, is, that at the 

 equator it rises higher in the heavens, and shines 

 more directly downwards. The farther away a 

 place is from the equator, the less is the average 

 noonday height of the sun, and therefore the 

 smaller the influence it exerts. 



The mean temperature of the equator is about 

 80. If the whole earth were a perfectly smooth 

 globe of one uniform kind of surface that is, if it 

 were all sea, or all one kind of level land the 

 temperature would decrease steadily from the 

 equatorial amount, according to the latitude ; but 

 the variations of the surface cause many important 

 deviations. 



The whole force of the sun's rays does not act 

 directly on the solid surface of the earth ; the 

 atmosphere, especially when charged with vapour, 

 arrests a certain portion of the heat, and is itself 

 rendered warm by the portion arrested. This is 

 one source of the warmth of the air. When clouds 

 are spread out in the atmosphere, the resistance 

 to the rays is still further increased, and a greater 

 portion taken up by the air itself. The other 

 sources of atmospheric heat are, contact with the 

 surface of the earth, and the radiation of heat from 

 the ground upwards. 



The air exercises a very important influence in 

 keeping up the mean temperature of the earth, by 

 resisting the passage of heat outwards, on the 

 same principle that our bodies are kept warm by 

 clothing. The thinner the covering of air, the 

 colder would the earth be ; as we see in ascending- 

 to the tops of mountains, at which the temperature 

 is always much lower than at the sea-level the 

 cold increasing with the height. These high places 

 receive a more intense solar radiation through the 

 thin covering of air that lies upon them ; but such 

 is the ease with which the heat can radiate off 

 through a thin atmosphere, that they are always 

 kept comparatively cold. If we had no atmosphere 

 at all, the rays of the sun would be very intense 

 where they actually struck, but so rapid would be 

 the loss of heat by radiation, that the whole earth 

 would be permanently kept far below freezing ; no 

 liquid material of any known kind could exist on. 

 its surface. 



