20 AGRICULTURE ON THE PACIFIC SLOPE 



by dissolving a little of the rock, enlarge these crevices 

 to cracks, which then gradually cleave off particles of 

 various sizes, and even bowlders. On mountains one 

 can sometimes take up rock powder in handfuls from 

 the surfaces of hard rocks. 



Heat an iron bolt, for which you have a well-fitting 

 nut, to dull redness ; then try to put on the nut quickly. 

 The nut will seem too small for the thread of the bolt 

 until the latter cools or the nut is allowed to heat on 

 the bolt. 



If a blacksmith's shop is near, watch the putting on 

 of a wheel tire. This is heated almost red-hot in a fire 

 and then it drops over the wooden wheel easily. On 

 cooling, it will fit the wheel so closely that it binds the 

 wooden rim firmly together. In the common mer- 

 curial thermometer the mercury expands faster than 

 the glass bulb, and so rises in the stem. This is true 

 of all liquids and still more of gases. Observe how 

 a white glass bottle when heated on the stove and then 

 inverted in water will suck its neck full of water from 

 the contraction of air, notwithstanding that the bottle 

 also shrinks, but much less than the air. 



Third, rocks are ground and powdered by streams of 

 water, which may start from the ends of glaciers, from 

 springs, from seepage water, or from the surface drain- 

 age of rains. The flowing water rubs and grinds the 

 various sizes of rock fragments, sand, or powder against 

 each other, forming rounded pebbles and cobbles on 



