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114 overcome if its velocity were not so increased. Under certain circumstances it would go out and come back again, and so on; that is the case with regard to the law of gravitation.

I will now take a few minutes only for the next section. We will depart from the consideration of mechanical forces, and consider the measure of distances. The thing which I wish to explain to you is, how can we measure the distance of the moon from the earth? The distance of the moon is measured by the method of Parallax. This is a technical word of which we are obliged to make perpetual use in Astronomy. I will explain in as few words, and in as familiar a manner as I can, what parallax is. There is an experiment pleasing and profitable, and which I have made in my youth, and which I have no doubt most of you have made in your time. It is this: if you place your head in a corner of a room, or on a high-backed chair, and if you close one eye and allow another person to put a lighted candle upon a table, and if you then try to snuff the candle with one eye shut, you will find that you cannot do it; in all probability you will fail nine times out of ten. You will hold the snuffers too near or too distant; you cannot form any estimation of the distance. But if you open the other eye the charm is broken; or if, without opening your other eye, you move your head sensibly, you are enabled to judge of the distance. I will not speak of the effect of motion of the head at present, but will call your attention to the circumstance, that when your head is perfectly still you will be unable, with a single eye, to judge with accuracy of the distance of the candle. In Figure 38, let A and B be the two eyes, C an object which is viewed first with the eye A only. This eye alone