FOREST DESTRUCTION IN NEW SOUTH WALES. 65 



knowledge sufficient to enable us to predict the seasons for a 

 considerable time, perhaps more than a year. 



I think it will be generally admitted that the total amount of 

 heat which the earth with its atmosphere receives from the sun each 

 year, is so nearly the same that there is no appreciable difference. 

 Unusually cold summers or warm winters at any one place may be 

 accounted for, and are accounted for by the varying distribution 

 of heat on the earth's surface, caused by atmospheric and oceanic 

 currents, but they do not affect the total of heat units. This 

 being so, it follows that the amount of work done by the sun's 

 heat falling on the surface of the earth, of lakes, of rivers, and of 

 the ocean, in raising water by means of evaporation, and holding 

 it suspended in the atmosphere, must be the same from year to 

 year. Our atmosphere at a given temperature, or to put it in 

 another form, charged with a given amount of heat, is only 

 capable of holding in suspension a certain fixed proportion of 

 water, so that after the point of saturation is reached, if the 

 temperature be reduced, some of the water in some form must 

 return to the surface of the earth or sea. 



Now if the quantity of heat received by the earth be the same 

 from year to year acting on the same surfaces of sea and land, 

 will it not raise by evaporation precisely the same amount of 

 water each year. If our atmosphere be only capable with the 

 same amount of heat each year of holding in suspension the same 

 quantity of water, is it not certain that there will be a like surplus 

 in the various forms of rain, hail, and snow, to return again to 

 the earth's surface each year. If this be admitted, it follows with 

 absolute certainty that the total rainfall of the earth's surface, if 

 under this designation we include all the forms in which water is. 

 deposited from the atmosphere, must be precisely the same from 

 year to year, or can only vary within scarcely appreciable limits. 

 Though we may not be able to say what the total rainfall of the 

 earth amounts to, we may be fairly sure that it does not vary 

 much if at all. This being so, it will follow that an excess of 

 rainfall — a flood year — in any one part of the World must produce a 

 deficiency of rainfall — a drought — in some other part of the World, 

 or it may happen that a very slight reduction of the general 

 rainfall may supply the excess which produces the local flood. 



We cannot tell which of these explanations is true at present, 

 but if the first be the correct one, we may possibly by comparing 

 the rainfall records for different parts of the World, and over long 

 periods of time, be able to find out in what particular places the 

 droughts and floods compensate each other, and in what order of 

 time; in fact, we may be able to say where our surplus rainfall 

 comes from when we have a flood year, and where the proportion 

 of our rainfall that is deficient has gone when we have a drought 

 year. Such knowledge, if attained, would perhaps enable us to 



E-June 6, 1888. 



