REVIEWS. 441 



the recurrent average conditions that we may expect, in successive 

 seasons, repeated year alter year, which we call climate." 



The above statement gives an idea of the scope and method of 

 treatment of the subject. There are a few points, however, which 

 deserve more particular mention. In chapter III., the distribution 

 over the earth of the insolation, or radiant energy received by the earth, 

 is discussed, and by means of a very ingenious diagram, the amount 

 of insolation for all latitudes for each month of the year is graphic- 

 ally shown. A detailed discussion of the various processes of absorb- 

 tion, conduction, radiation, and convection, by means of which the 

 atmosphere gains and loses heat, is given. In the course of this the 

 author takes exception to the statement, so common in most physical 

 geographies, in which the atmosphere is "compared to a trap which 

 allowed sunshine to enter easily to the earth's surface, but prevented 

 the free exit of radiation from the earth." In reality, the coarse- 

 waved radiation from the earth passes out readily without great absorb- 

 tion, either by the clear air or the water-vapor, which has been proved 

 to be as poor an absorber as pure dry air. 



Again, the exact processes, by which convectional circulation is set 

 up, are clearly brought out, and the incorrectness of such loose state- 

 ments, as " the air is heated and rises, and the cold air rushes in from 

 either side to fill the vacuum thus formed," is emphasized. 



A general review of the distribution of pressures and the circula- 

 tion of the winds shows the student two particulars, in which the 

 expected arrangement of pressures and motions according to the 

 theory of convection, as applied to the origin of winds, are contra- 

 dicted by the facts. The polar pressures are high, not low, the high- 

 est pressures occur around the tropics, where intermediate pressures 

 were expected, and the winds do not follow the gradients, but are 

 systematically deflected. Either the convection theory is fundament- 

 ally wrong as an explanation for the winds, or it needs to be supple- 

 mented by some factors up to this time unconsidered. This fact the 

 author brings clearly to the mind of the pupil, who is then led to see 

 that, perhaps, the oblique course of the winds may account for the 

 distribution of pressures at the poles and the tropics. The cause 

 of the oblique course is found in the deflecting influence of the 

 earth's rotation. It is proportionate to the velocity of motion, and 

 increases from zero at the equator to a maximum value at either pole, 

 but it does ?iot depend upon the direction in which the body is mov- 



