The heat of the sun beam goes through glass without [Page 34] any hinderance whatever.  It streams into the room as freely as if there were no glass there.  But what if the furnace or stove heat went through glass with equal facility?  We might as well try to heat our rooms with the window-panes all out, and the blast of winter sweeping through them.

The heat of the sun, by its intense vibrations, comes to the earth dowered with a power which pierces the miles of our atmosphere, but if our air were as pervious to the heat of the earth, this heat would flyaway every night, and our temperature would go down to 200 deg. below zero.  This heat comes with the light, and then, dissociated from it, the number of its vibrations lessened, it is robbed of its power to get away, and remains to work its beneficent ends for our good.

Worlds that are so distant as to receive only 1/1000 of the heat we enjoy, may have atmospheres that retain it all.  Indeed it is probable that Mars, that receives but one-quarter as much heat as the earth, has a temperature as high as ours.  The poet drew on his imagination when he wrote: 

  “Who there inhabit must have other powers,
   Juices, and veins, and sense and life than ours;
   One moment’s cold like theirs would pierce the bone,
   Freeze the heart’s-blood, and turn us all to stone.”

The power that journeys along the celestial spaces in the flashing sunshine is beyond our comprehension.  It accomplishes with ease what man strives in vain to do with all his strength.  At West Point there are some links of a chain that was stretched across the river to prevent British ships from ascending; these links were made of two-and-a-quarter-inch iron.  A powerful locomotive might tug in vain at one of them and not stretch it the thousandth part of an inch.  But the heat of a single gas-burner, that glows with the preserved sunlight of other ages, when suitably applied to the link, stretches it with ease; such enormous power has a little heat.  There is a certain iron bridge across the Thames at London, resting on arches.  The warm sunshine, acting [Page 35] upon the iron, stations its particles farther and farther apart.  Since the bottom cannot give way the arches must rise in the middle.  As they become longer they lift the whole bridge, and all the thundering locomotives and miles of goods-trains cannot bring that bridge down again until the power of the sunshine has been withdrawn.  There is Bunker Hill Monument, thirty-two feet square at the base, with an elevation of two hundred and twenty feet.  The sunshine of every summer’s day takes hold of that mighty pile of granite with its aerial fingers, lengthens the side affected, and bends the whole great mass as easily as one would bend a whipstock.  A few years ago we hung a plummet from the top of this monument to the bottom.  At 9 A.M. it began to move toward the west; at noon it swung round toward the north; in the afternoon it went east of where it first was, and in the night it settled back to its original place.