136 YALE AGRICULTURAL LECTURES. 



But the temperature is not the same for places in the same lat 

 itude in the two hemispheres, as is seen in the following table : 



PLACES. LAT. TEMP. PLACES. I.AT. TUMP. 



Falkland Isles, 51 S 47 23 London, 51 31' N 50 -72 



Buenos Ayres. 34 36' S 62 6 Savannah, 32 05' N" 64 -58 



Rio Janeiro, 22 56' S 73 96 Calcutta, 22 35' N 78 '44 



This variation is owing to a variety of local causes, such as 

 the elevation and form of the land, proximity to large bodies 

 of water, the general direction of winds, etc. v 



The temperature of the air diminishes with the altitude. As 

 a general rule, it may be stated that there is a diminution in 

 temperature of 1 F. for every 343 feet of elevation. On ris 

 ing from near the level of the sea, the rate of decrease is more 

 rapid ; after a certain height is reached it proceeds more 

 slowly ; but in very elevated regions it again increases. 



It follows from this that in every latitude, at a certain eleva 

 tion, there must be a point where moisture once frozen must 

 ever remain congealed. The lowest point at which this is at 

 tained is called the limit of perpetual snow, or the snow-line. 

 This point is highest near the equator, and sinks towards either 

 pole, as is shown in the table. 



PLACES. LATITUDE. SNOW LINES. 



Straits of Magellan, 54 S 3,760 feet. 



Chili, 41 S 6,009 



Quito, 00 15,807 " 



Mexico, 19 N 14,763 " 



^Etna, 37 30' N 9,531 " 



Kamtschatka, 56 40' N 5,248 " 



Isothermal lines were very briefly illustrated from a map of 

 the United States, on which were traced from the map in the 

 Patent Office Report for 1856-7, the lines of summer and win 

 ter temperature in various latitudes. The great value and 

 importance of such researches to agriculture were insisted on 

 by the lecturer as giving the only rational explanation to 

 anomalies of climate, etc., otherwise inexplicable. The great 

 contrast between the latitudes and isothermes of wheat and 



