258 MANUAL OF AGRICULTURE. 



colder surface of the western parts of our islands; they be- 

 come lowered in temperature, and consequently part with 

 much of their moisture at the place of contact, in the form of 

 mists and rain ; and thus they keep these parts at once warmer 

 and more humid than the eastern districts of the kingdom. 

 Such influences have, hy necessity, a considerable bearing 

 upon the question as to the preferable system of agriculture for 

 adoption in these districts respectively. Tor whilst the posses- 

 sion of an atmosphere humid to excess, with its attendant 

 want of frequent sunshine, renders certain districts incapable of 

 properly maturing our more valuable cereal crops, they are at 

 the same time better fitted for the cultivation of the important 

 green crops, and thereby better calculated for the successful 

 practice of the several systems of stock-farming than other, in 

 some respects, more highly favoured counties. 



The following remarks of the English Eegistrar-General are 

 of interest in this connection. He says : — " Eain fell in London 

 to the amount of 43 inches, which is equivalent to 4300 tons 

 of rain per acre. The rainfall during last week" (February 

 1865) " varied from 30 tons per acre in Edinburgh, to 215 tons 

 per acre in Glasgow. An English acre consists of 6,272,640 

 square inches, and an inch deep of rain on an acre yields 

 6,272,640 cubic inches of water, which at 277,274 cubic inches 

 to the gallon makes 22,622'5 gallons ; and as a gallon of dis- 

 tilled water weighs 10 lbs., the rainfall on an acre is 226,225 lbs. 

 avoirdupois ; but 2240 lbs. are a ton, and consequently an inch 

 deep of rain weighs 100*993 tons, or nearly 101 tons per acre. 

 For every 100th of an inch, a ton of water falls per acre. If any 

 agriculturist were to try the experiment of distributing artifici- 

 ally that which nature so bountifully supplies, he would soon 

 feel inclined to rest and be thankful." 



Numerous experiments have satisfactorily demonstrated that 

 the amount of water exhaled by the plants on an acre of ground 

 is in excess of its amount of rainfall. As therefore nearly 2-5ths 

 of the total rainfall are carried away by the drainage, it will be 

 better judged to what an extent takes place an almost insensible 

 circulation of water from earth to atmosphere and reversely. 



The atmospheric temperature decreases as we approach the 

 higher latitudes from the equator, and also with increased eleva- 

 tion above the sea-level. For every 300 feet of ascent above the 

 sea-level the mean or average temperature decreases 1°. But 

 with increasing distance from the equator there is no uniform 

 gradient of decrease, owing to the unequal distribution of sea 

 and land and other causes producing a variation of temperature 

 in parts of the world included in the same degrees of latitude. 

 The lines which cover tracks of the world having the same mean 

 temperature are caUed "isothermal lines." The atmospheric 



