542 



METEOROLOGY. 



thickly coated on the under than on the upper 

 side. Mr. Aitken's examinations of roads con- 

 tradicted the genera] opinion that dew is not 

 formed in such places, and gave support to his 

 theory ; for while dew was not visible on the 

 surface of the roads, the under sides of the 

 stones of the gravel were found dripping wet. 

 Slates placed over the gravelly part of the road, 

 and also over a hard, dry part, were likewise 

 found dry on top and wet beneath. Examina- 

 tions of the different forms of vegetation led 

 to the conclusion that the large drops of moist- 

 ure, which appear at the tips of the lelaf-blades, 

 were not dew, but exudations, effects of the 

 vitality of the plant. Mr. A. T. Fraser, of 

 India, has found in Mr. Aitken's theory an 

 adequate explanation of the phenomenon, often 

 observed, of the luxuriant vegetation that ap- 

 pears where tools are left to lie upon the grass. 

 The outer surfaces of the iron parts radiate 

 quickly at night, and the stratum of air inclosed 

 between the metal under surface and the earth 

 is therefore saturated with condensing moist- 

 ure. 



Clouds. Mr. A. W. Clayden draws the con- 

 clusions, from measurements made during the 

 summer of 1885, that clouds of less than 2,000 

 feet in thickness are not often accompanied by 

 rain, and if they are, it is only very gentle, con- 

 sisting of minute drops. With a thickness of 

 between 2,000 and 5,000 feet, the size of the 

 drops is moderate. As the thickness augments, 

 the size of the drops increases, and their tem- 

 perature becomes lower, until, when the thick- 

 ness is upward of 6,000 feet, hail is produced. 

 The author points out that rJl observations tend 

 to show that, except under abnormal condi- 

 tions, the temperature of the atmosphere falls 

 as the height above the sea-level increases ; and 

 there seems no reason for assuming that the 

 law does not apply to that portion of the at- 

 mosphere which forms a cloud. Hence, if a 

 drop were to be formed at or near the upper 

 surface of a cloud, it would fall down into a 

 region saturated with vapor at a temperature 

 above its own. The result will be further con- 

 densation, producing a larger drop; and this 

 process will continue until it leaves the cloud. 

 If its temperature is below the dew-point of 

 the air, it falls through ; condensation will con- 

 tinue until it reaches the ground. This subse- 

 quent gain can not, however, bear any very 

 large proportion to the growth while falling 

 through the saturated cloud; hence, the con- 

 . elusion follows that the size of the drop must 

 increase with the thickness of the cloud. The 

 author suggests that condensation begins on 

 the upper surface of the cloud by the cooling 

 of some of the liquid cloud-particles. If this 

 particle is cold enough it will solidify, and snow 

 will be formed. Should it not be quite cold 

 enough to solidify at once, owing to its minute- 

 ness, but remain still below the freezing-point, 

 hail is formed. Finally, if the temperature is 

 not low enough for either snow or hail, rain is 

 produced. 



Rainfall. Mr. John Murray has made an es- 

 timate of the extent of the areas of the differ- 

 ent mean annual rainfalls over the globe. The 

 amount of rain that falls upon the globe an- 

 nually is estimated at from about 34,000 to 35,- 

 000 cubic miles. Taking the inland drainage 

 areas disconnected with the sea, such as the 

 Sahara Desert, it is found that 77 cubic miles fall 

 upon those surfaces, which must be regarded 

 as equivalent to the amount of evaporation. 

 He cites the American calculation that 99,000,- 

 000 millions of cubic feet fall annually over 

 the Mississippi drainage area. Estimating the 

 outflow of the river, it appears that only one 

 fourth of that water reaches the ocean. By 

 extending the inquiry over large areas, it is 

 hoped to make it of some practical importance. 



In investigating the rainfall of the British 

 islands for the 24 years, 1860 to 1883, according 

 to the account given by Mr. Alexander Buchan, 

 observations were examined from 1,840 sta- 

 tions. The results were marked upon the map 

 so as to show six classes of regions or divisions, 

 namely, those in which the annual rainfall did 

 not amount to 25 inches, and in which it was 

 from 25 to 30, 30 to 40, 40 to 60, and 60 to 80 

 inches, respectively. The regions of heaviest 

 rainfall comprised the greater part of Skye and 

 a large portion of the mainland to the south- 

 east, the greater part of the lake district, a part 

 of the more mountainous portion of North 

 Wales, and the mountainous district of the 

 southeast of Wales. Of these, the west High- 

 lands present the most extensive region, and 

 offer a practically unbroken wall directly in the 

 course of the rain- bringing winds from the At- 

 lantic. But south of this region, from Luss to 

 the lake district, the rainfall nowhere rises to 

 80 inches; the diminution is ascribed to the 

 interposition of Ireland in the path of the At- 

 lantic winds. St. George's Channel and the 

 Irish Sea open a free passage to the southwest- 

 erly winds, which are here diverted into a more 

 southerly course, to the north of England and 

 Wales; and accordingly, when the mountain 

 masses of the lake district and Wales oppose 

 their course, the maximum rainfall is again 

 reached. The largest region of 60 to 80 inches 

 rainfall is in the west Highlands, surrounding 

 the region of 80 inches and upward. An an- 

 nual rainfall of from 40 to 60 inches covers ex- 

 tensive areas of the islands ; the rate of at least 

 40 inches characterizing the climates of about 

 a fourth part of the surface of England, about 

 half of Ireland, and considerably more than 

 half of Scotland, which latter, taken as a whole, 

 is by far the rainiest of the three divisions of the 

 United Kingdom. The breakdown at various 

 intervals of the hilly plateau, which stretches 

 along the west of Great Britain, has a striking 

 influence on the distribution of the rainfall. 

 Thus the opening of the Bristol Channel fur- 

 nishes an avenue for a more generous watering 

 of a large portion of central England, tK 

 breakdown of the plateau between the Pennine 

 range and North Wales, and the lowering of 



