180 BOARD OF AGRICULTURE. 



The Washington elm at Cambridge, a tree of no extraor- 

 dinary size, was some years ago estimated to produce a crop 

 of 7,000,000 leaves, exposing a surface of 200,000 square 

 feet, or about five acres of foliage. 



Generally during summer, and especially if the season be 

 a dry one, plants absorb from the earth all the moisture 

 their roots can reach ; and here is another element to bo con- 

 sidered in connection with the fact that plants exhale moist- 

 ure in proportion to their amount of foliage and luxuriance 

 of growth. The roots of a scanty pasture grass extend 

 downwards not many inches ; of corn, they have been found 

 six or seven feet below the surface, and tree roots have been 

 found twenty-five feet below the surface. The deeper the 

 roots, the larger the proportion of water absorbed by them 

 and exhaled into the air by their leaves, and the less the 

 proportion that escapes to the deep subsoil, and that finally, 

 by springs, brooks and rivers, finds its way to the ocean. 



It has been observed that forests draw rain. The atmos- 

 phere always contains more or less moisture ; the higher the 

 temperature the more it can contain. If, when saturated, its 

 temperature be lowered it must part with a portion of its 

 moisture in the shape of rain, dew, frost, snow or hail. If 

 the air is not saturated it is constantly taking up more 

 moisture by absorption from the soil, vegetation, water or 

 snow on the surface of the earth. To give an illustration : 

 As the trade- winds blow constantly across the tropical ocean 

 west of Africa, the air becomes saturated, and on reaching 

 the rising land and cooler air of the South American conti- 

 nent gradually parts with its moisture, until it finally reaches 

 the eastern slope of the Andes, which rises up like a huge 

 barrier, and demands, by virtue of its colder air, almost 

 the last tithe of moisture it has received from the ocean. 

 On passing over the summit of the range and down the 

 western slope of the mountains the conditions are reversed. 

 The gradually increasing temperature is far above the dew- 

 point of the dried atmosphere, and gives it a constantly 

 increasing capacity for moisture, and it demands and receives 

 of the earth all it can possibly exhale. The rains of the 

 eastern slope give rise to the largest rivers in the world, the 

 Amazon and its tributaries ; while on the western slope is 



