METEOROLOGY. 



18& 



ground from the extrtnuo cold of winter. In very eold MMOM 

 the temperature of the ground under the anow in often many 

 degree* higher than that of tho air around ; tint iinportanoe of 

 thU to tho young plants oan easily be underatood. 



seema to be caused by miow-nakos becoming partially 

 melted in their fall. The snow is sometimes produced in tho 

 in^ii.-r regions of the atmosphere, being caused by a current of 

 air ut a very low temperature. Tho air below ia, however, 

 .1 dogreea wanner, and hence tho flukoH are partially 

 melted in fulling, and as they oome in oontact with one an- 

 i tli.-r, unite, and thus produce tho larger Hakes so frequently 

 noticed. 



Sometimes tho moisture of tin air fall.i to tho ground in the 

 shape of small pieces of ioe, known as hail. There ia some 

 .iitli.'ulty in accounting fully for this, especially as it is most 

 common in tropical regions and during the heat of summer. 

 It usually falls, too, at the hottest part of the day. 



Its formation is probably to be explained by a very cold 

 current of air suddenly rushing into one at a much higher tem- 

 perature, and nearly saturated. Tho rain thus formed is at once 



veaael down into the anow till it tonchee the ground, 

 take it up with the anow in it, and, aetting it in a warm 

 let it Htand till the anow ia melted. Tho depth of water can 

 then be meaaured, and thia amount ahoold be added to the rain. 

 fall of the day, aa ahown by the gauge. 



Waterspouts and whirlwind* oonatitate another very remark- 

 able olaaa of meteoric phenomena, which we will notice here, 

 though they aeem to belong partly to aerial and partly to 

 aqueous meteors. 



When two currents of air are impelled obliquely againat one 

 another, a whirlwind or eddy ia produced. Theae may eon* 

 stantly be seen on a small scale during summer, when the gnata 

 of wind will catch up email particles of straw, dust, or looae 

 materials, and whirl them round in a spiral, sometimes carrying 

 them a little way up in the air. 



In sandy deserts this is witnessed on a much grander scale. 

 Tho sand is caught up by the wind, and formed into tall, stout 

 columns, which rotate on their axes, and move about, sometime* 

 with a slow, stately motion, at other times travelling very 

 rapidly, and in irregular directions. The appearance presented 



11. DISCHARGES OF ELECTRICITY DURING A VOLCANIC ERUPTION. 



congealed into small lumps of ice. Thia theory is supported by 

 tho greater frequency of hail-storms in mountainous regions, 

 where these cold blasts are produced by the proximity of fields 

 of snow and ice. 



Hailstones are usually of a globular form, and regular in 

 shape. If they be cut through, their internal structure is likewise 

 found to be regular, as if composed of crystals radiating from a 

 centre. They vary in size from about a tenth of an inch upwards 

 to several inches in diameter. Many well-authenticated instances 

 are on record in which they have attained the size of pigeons' 

 eggs, and they have been known much larger. 



In many places much damage is done to the crops by hail- 

 storms, tho tender plants being beaten down and destroyed. In 

 the south of France the annual damage to tho vines from this 

 cause is very great and serious. Sometimes birds, and even 

 larger animals, have been killed by the hail, and great injury 

 has been caused to windows and roofs of houses. 



The fall of snow or hail is not indicated by a rain-gauge, and 

 hence the reading of this does not represent the full amount of 

 water that falls upon the ground. Usually snow is found to 

 occupy some ten or twelve times the bulk of an equal weight of 

 water ; but when the temperature is high it lies closer than this. 

 Some observers merely measure the depth of the snow in a place 

 where it is not affected by drifts, and then take one-tenth of 

 this to represent the equivalent rain-fall. This is not, however, 

 a very accurate plan ; the better way is to plunge an open tin 



by a large number of these sand-columns has been described by 

 various travellers, and is, indeed, very remarkable. 



The dust whirlwinds of India are similar phenomena. Their 

 presence is first indicated by a dark cloud seen in the horizon ; 

 this rapidly spreads, and bursts upon the observer. The air is 

 quite thick with the dust which is carried up by the wind, and 

 sometimes the storm may be seen advancing, and presenting 

 the appearance of a number of spiral clouds hurrying rapidly 

 onwards. The air is highly electrical, this being probably pro- 

 duced by friction of the particles of dust in the air. 



Sometimes the agitation of the air becomes even more violent, 

 and then we have the tropical whirlwinds, which uproot trees, 

 overturn buildings, and seem to carry everything before them. 



When those whirlwinds occur over large bodies of water, they 

 often give rise to waterspouts. A dark cloud appears, and is 

 whirled round by the wind so as to produce a conical mass of 

 vapour, reaching nearly to the surface of the water, which is 

 likewise so violently agitated that frequently it appears to be 

 connected with the lower end of the vapoury column. Many 

 have supposed that the water is actually sucked up from the sea 

 by the spiral motion, but this is now known to be a fallacy, aa 

 water that falls from the spout on the surface of vessels is 

 found to be fresh, and not salt, aa it would be were tliis theory 

 true. 



A great quantity of rain is often produced as those waterspouts 

 break. Sometimes they travel on to the land, and the mass of 



