Xocal TOn&s, 101 



friction caused by contact of the air with the 

 earth's surface, but more particularly by trees, 

 inequality of surface, and other obstructions 

 on the earth. 



There is a variety of wind called mountain 

 winds that arise from different causes. As 

 has been stated in a former chapter, under 

 ordinary conditions the air is more dense at 

 sea-level than at any point above, and the den- 

 sity is constantly changing from denser to 

 rarer the higher we ascend. Suppose at a cer- 

 tain point, say halfway up a mountain side, 

 the air has a certain density, and if it is at 

 rest the lines of equal density or pressure will 

 seek a level, just as water would under the 

 same conditions. Suppose we start at a given 

 point on the side of a mountain and run out 

 on a level till we are 100 feet in a perpendicu- 

 lar line above the side of the mountain, the air 

 contained within those lines will be in the 

 shape of a triangle. If now the sun shines 

 upon the side of the mountain the air is 

 warmed and expands according to a well- 

 known law, and the amount of expansion will 

 depend upon the depth of the volume of air; 

 hence the point of greatest expansion in our 

 figure will be where the air is 100 feet deep, 

 and will gradually decrease as we go toward 

 the mountain till we come to the point where 

 our horizontal line makes contact with the 

 mountain side. At that point, of course, 



