190. =A Comparison.=--The above can be made more clear and impressive by a familiar comparison. A man on board of a steamboat, by confining his attention to things within the boat, may, after a while, be almost unconscious of the boat's moving, if the water be smooth, though the boat may be going at the rate of fifteen miles an hour. If he be reading in the cabin he will think as little of his motion as he would were he reading in his parlor at home. If he should be blindfolded, and turned around a few times, it would be impossible for him to tell the direction in which the boat is going. Now it is with a man on the earth as it is with the man in the boat. He is unconscious of the motion of the earth for the same reason that the man in the boat is unconscious of the boat's motion. All objects around him are moving along with him, as the objects around the man in the cabin of the boat are moving along with him. We can carry the parallel farther. While the man sits in the cabin he knows not how fast the boat moves, nor even whether it moves at all. He must look out to decide this, and even then he may not be able to tell whether the boat moves, or whether he merely sees the water running by it. We often are actually deceived in this respect. A steamboat struggling against wind and wave may appear to those on board to be advancing when it is really stationary, or even when it is losing ground. So when we look at the sun we know not whether it is the sun or the earth that is moving. Mere vision, without reasoning on the subject, leads one to think that it is the sun that moves. For the same reason, if a child should be placed in a carriage for the first time without seeing the horses, but with its eyes fixed on objects at the road-side, he would probably think that all the fences and trees and rocks and houses are in motion.