CHAMBERS'S INFORMATION FOR THE PEOPLE. 



n brisk south-west or south-south-west wind blows 

 between the equator and the southern limit of the 

 northern trade, which is then at about 10 or 12 

 north latitude. In America, also, similar breezes 

 are experienced. 



Rainfall.* The chief sources of evaporation 

 are the seas, lakes, and rivers. Although there 

 is three times as much sea as land, this is not 

 more than enough to keep up a sufficient mois- 

 ture for the habitable countries ; for although 

 some regions have rather more than is desirable, 

 many large tracts of country remain desert and 

 uninhabitable solely from the dryness of their air 

 and the scarcity of rain. 



The evaporation is necessarily greatest in the 

 equatorial regions, where the temperature is 

 greatest, and decreases steadily towards the 

 poles. And although a great deal of the tropical 

 evaporation is transported by air-currents into 

 higher latitudes, the precipitation of vapour, or 

 the rain, is also most abundant in the warmest 

 climates, and in the neighbourhood of the trop- 

 ical seas. 



Rain is the most capricious of all the meteor- 

 ological phenomena, both as regards its frequency 

 nnd the amount which falls in a given time. It 

 rarely or never falls in certain places, which are, 

 on this account, designated the rainless regions 

 of the globe the coast of Peru, in South America ; 

 the great valleys of the rivers Columbia and Colo- 

 rado, in North America ; Sahara, in Africa ; and 

 the Desert of Gobi, in Asia, are examples ; whilst, 

 on the other hand, in such places as Patagonia, 

 it rains almost every day. Again, the quantities 

 which have been recorded at some places to have 

 fallen at one time, are truly enormous. In Great 

 Britain generally, if an inch fall in a day, it is 

 considered a very heavy rain. In many parts 

 of the Highlands of Scotland, three inches not 

 unfrequently fall in one day. On the 5th of 

 December 1863, there fell at Portree, in Skye, 

 12^ inches in 13 hours. At Seathwaite, in 

 Borrowdale, 6-62 inches fell on November 27, 

 1845. But it is in continental, and especially 

 tropical countries where the heaviest single 

 showers have been recorded. The following are 

 a few of the most remarkable : At Joyeuse, in 

 France, 31*17 inches fell in 22 hours ; at Geneva, 

 30 inches in 24 hours ; at Gibraltar, 33 inches in 

 26 hours ; on the hills above Bombay, 24 inches 

 in one night ; and on the Khasia Hills, 30 inches 

 on each of five successive days. 



The heaviest annual rainfall on the globe is 600 

 inches on the Khasia Hills, about 500 inches of 

 which falls in seven months during the south- 

 west monsoons. This astonishing amount is due 

 to the abruptness of the mountains which face the 

 Bay of Bengal, from which they are separated by 

 200 miles of low swamps and marshes. 



The following are some of the annual rainfalls 

 in the tropics : Singapore, 97 inches ; St Benoit 

 {Isle of Bourbon), 163 inches ; Sierra Leone, 87 

 inches; Caracas, 155 inches; Pernambuco, 106 



* The quantity of rain which falls at any station during a given 

 time is ascertained by means of the rain-gauge an instrument 

 constructed in various ways. One of the simplest forms consists 

 of a cylindrical copper vessel furnished with a float : the rain 

 falling into the vessel raises the float, the stem of which is so 

 graduated that an increase in depth, to the extent of one-hundredth 

 of an inch, can be ascertained. It is observed that rain-gauges 

 collect different quantities at different heights above the ground, 

 the amount being always greater at the lower level. This remark- 

 able fact has never been quite accounted for. 

 40 



inches ; Rio Janeiro, 59 inches ; Georgetown, 100 

 inches ; Bahamas, 52 inches ; and Vera Cruz, 183 

 inches. 



In general, when a mountain-chain crosses the 

 course of an ocean wind, the weather-side is wet 

 and the lee-side is dry. The trade-winds of the 

 Atlantic sweeping over the continent of South 

 America become cooled as they ascend towards 

 the Andes, and, depositing their vapour, feed the 

 mighty streams of the Amazon and Orinoco. 

 On the west side of the wall of the Andes, in 

 Peru, rain is almost unknown. 



Distribution of Atmospheric Pressure. The 

 varying pressure of the atmosphere, from place 

 to place and from time to time, is the chief 

 cause of all fluctuations of wind and weather. 

 A knowledge, therefore, of the distribution of this 

 pressure over the globe, and the laws of its 

 changes, as indicated by the barometer, is the 

 basis of meteorology. Variations of the barom- 

 eter are of two kinds periodical and irregular, 

 Of the regularly recurring variations, the daily 

 variation is of small amount, and of little direct 

 importance. The phenomenon has its cause in 

 the daily march of the sun round the globe. 

 The annual variation follows the march of the 

 sun from one side of the equator to the other. 

 The greater power of the sun in the northern 

 hemisphere in summer rarefies the air, and 

 causes a transference of a part of it into the 

 colder southern regions, thus diminishing the 

 pressure. The presence of a greater proportion 

 of vapour acts in the same direction, owing to 

 vapour having less specific gravity than dry air. 

 As a rule, then, the atmospheric pressure is least 

 in the summer, and greatest in the winter, in each 

 hemisphere. But this annual fluctuation is subject 

 to much greater local differences and anomalies 

 than is the daily. Thus, in Siberia, the pressure 

 is ^ths of an inch less in July than in January. 

 The great heat of Siberia in summer causes the 

 air to expand and flow away in all directions. 

 In winter, on the other hand, the great cold and 

 small rainfall of that region cause high pressures 

 to prevail. In Iceland and Orkney, again, the 

 reverse of all this holds : the pressure is least 

 in winter, and greatest in summer. The summer 

 temperature of the North Atlantic, in which these 

 islands are situated, is cool as compared with the 

 heated continents that enclose it, hence there is 

 an overflow from these regions into the basin of 

 the North Atlantic, and therefore increased pres- 

 sure ; while in winter, the temperature there is com- 

 paratively high, causing an overflow of air towards 

 adjoining countries, and a diminished pressure. 



The distribution of atmospheric pressure is best 

 exhibited by means of isobarometric charts; that 

 is, charts on which lines are drawn through all 

 places where the height of the barometer is the 

 same. Such a chart may represent the mean 

 pressure for the year, or the mean for a portion 

 of the year, such as a particular month ; or the 

 readings of the barometer at a specified moment, 

 such as the beginning or middle of a storm. 

 Owing to the immense labour of collecting and 

 arranging the hundreds of thousands of observa- 

 tions necessary for such a purpose, it is only 

 recently that the attempt has been made. Mr Alex- 

 ander Buchan, Secretary of the Scottish Meteorol- 

 ogical Society, has the merit of first undertaking 



