higher up, its rays are not so much refracted as in the case of the Sun. The altitude of the heavenly bodies seems greater, therefore, than it really is, in proportion as we come down to the horizon, where the air is thicker and more humid. The effects of refraction were first noticed by Ptolemy, the Egyptian astronomer, about the year A.D. 140.

While the Sun is within 18° of the horizon, or for an hour and twelve minutes before sunrise and after sunset, we have some light from the Sun; and for many nights about the time of the Summer solstice, the Sun being within 18° of the horizon, we have no actual night.

It follows, therefore, curiously as it may seem, that, when we seem to see the Sun just above the horizon at sunrise and sunset, he is not actually above the horizon, but only seems so, in consequence of the bending round of his rays towards the parts where we may be*. Again, the thickness of the stratum of horizontal air makes the Sun and Moon appear larger than when their rays come down to us less obliquely; and, owing to the difference of the density of the air at the upper and lower parts of the disks of the Sun and Moon, these bodies are often seen of a compressed or oval form.

* It was in consequence of this effect of refraction that on the 20th of April, 1837, the Moon appeared to rise eclipsed, before the Sun had set: which, but for refraction, could not possibly have happened; for the three bodies would not have been in a line.

M

WE must now recur to the consideration of the direct influence of the Moon upon our globe, and give our attention to the third class of phenomena which we enumerated at page 114, as resulting from the joint operation of the Sun, Moon, and Earth—the TIDES.

In our second chapter, we stated that one of the properties of matter is to attract other matter towards itself: for example, that a stone drops to the Earth, by virtue of the Earth attracting the stone; that the planets retain their wonted distances from one another, and from the Sun, by virtue partly of the operation of this law.

Now it will not be difficult to conceive that, if attraction has reference to all matter, and if the Earth and Moon attract each other reciprocally, the Moon's attraction, if not felt to influence the solid matter of the Earth, may produce a great effect on that form of matter which is most easily moved, i.e., the liquid form. Now, if we exert an attractive or moving power of any kind, on a solid mass, such as of iron, the whole mass will be moved at once, if any part be moved; because the particles of which it is composed are so closely and intimately bound together, that a pressing or pulling

force will not separate them; unless, indeed, it be very enormous. With a liquid, however, we know that such is not the case. If we move the finger rapidly to and fro in a basin full of water, we shall separate some of the water from the remainder, and propel it over the edge of the basin: we can dip a vessel into the water, and take out a portion of it, without altering the nature or appearance of that which remains. A solid body may be immersed in the liquid, so as to be completely surrounded by it; thus showing the ease with which the bulk of water may be separated, so as to allow other bodies to insinuate themselves between its particles. Nothing of this kind happens with solid bodies; and this difference is the sole cause of the production of Tides, which we proceed to explain.

The reason why the Earth revolves round the Sun, instead of round any of the planets, is, that the large mass of the Sun exerts a stronger attraction than any of the planets. Again, the reason why the Moon revolves round the Earth, is, that she is much nearer to the Earth than to any other planet; while at the same time she is much smaller than the Earth.

The distance between the Earth and the Moon is regulated by the amount of the attraction of one for the other, and by the velocity of the Moon's motion; and as those quantities are equal year after year, there is no perceptible disturbance exerted by one on the other. This is, however, only true with reference to the Earth, considered as a whole: we must bear in mind that three-fourths of her surface is covered with water, a substance more likely to be influenced by an attractive force than a solid body, owing to the ease with which the particles of water glide over one