382 THE CLIMATE OF IRELAND, AND 



of a year is such as would suffice to melt a coating of ice, covering 

 the whole glohe, and 101 feet in thickness. This great amount is, 

 however, very unequally distributed. The heating power of the 

 sim's rays depends, according to a very simple law, on their 

 ohliquity; but an elaborate analysis is required to calculate 

 the total quantity of heat received, during any given time, at a 

 given place. This much, however, it is easy to see namely, that 

 so far as solar heat alone is concerned, the mean temperature will 

 be greatest at the Equator, and diminish regularly as the latitude 

 of the place increases. 



But there are causes on the earth itself which interfere with 

 this regularity of distribution. If the surface of the globe were 

 all dry land, composed of the same kind of rock or soil, and 

 unchequered with vegetation, the law above referred to would 

 accurately hold, and the lines of equal mean temperature the 

 isothermal lines, as they are called would be all parallel to the 

 Equator. This, however, is far from being the fact. The land, 

 you know, is in part covered with the waters of the ocean, and in 

 part dry ; while the surface of the latter is endlessly varied in its 

 own nature, and in that of its vegetable covering. These causes 

 produce an inequality in the action of the sun's rays in places 

 having the same latitude, which will be readily understood. The 

 same amount of heat which would raise the temperature of the 

 rock, or soil, through any given number of degrees, will raise the 

 temperature of an equal weight of water through a much smaller 

 amount, on account of what is called its greater capacity for heat. 

 And, in addition to this, a very large portion of the heat which is 

 absorbed by the water is employed, not in raising its temperature, 

 but in changing its state into that of vapour. It becomes thus 

 what is called latent, and does not affect the thermometer or the 

 sense. Every pound of water converted into vapour takes in, in a 

 latent form, as much heat as would raise the temperature of 960 

 pounds through one degree. And all this heat is carried away by 

 the vapour, to reappear at some remote place, where the vapour is 

 converted into rain. 



Owing to these and other causes, the surface of the ocean is 

 never raised to so high a temperature as that of the dry land. It 

 rarely exceeds 85 Fahrenheit, even within the Tropics, while the 

 surface of the soil in the same region is sometimes heated to 140, 

 and upwards. Captain Sturt relates that, in parts of Australia, a 



