Since the Earth revolves round its axis once in 24 hours these tidal waves go round the Earth and as there are two tidal waves each day there are two high tides and two low tides at every place on the ocean, but these are chiefly noticeable on the coasts. Fig. 112 shows the position of the Moon and the Earth, while the wavy lines around the Earth represent the oceans. =Spring Tides and Neap Tides.=—Not only does the Moon’s attraction cause the tides, but the Sun’s pull on the Earth also produces tides. When the Sun and Moon are in a line with the Earth they pull together and the tides are raised very high and they fall very low, and these high and low tides are called _spring tides_. Since we have either new Moon or full Moon, when the Sun, Earth and Moon are in a line, the spring tides occur at these times, or twice every month. Figs. 113 and 114 show how the spring tides are formed. [Illustration: FIG. 113.—HOW SPRING TIDES ARE FORMED.] [Illustration: FIG. 114.—HOW SPRING TIDES ARE FORMED.] When the Moon is at its first quarter, as in Fig. 115, or in its last quarter, as in Fig. 116, the Sun’s pull and the Moon’s pull oppose each other and the high tides are lowest. These low high tides, which take place twice a month, are called _Neap tides_. =A Trip to the Moon.=—Many stories have been told about imaginary trips to the Moon and what the adventurers saw after reaching their destination. Our story is made up of facts and the only thing we shall imagine is that we have made the trip and set foot somewhere on the Moon. Having performed this mental somersault we shall carefully leave out all the hard questions about living there without air and water, for this is a pleasure trip and we don’t want to spoil the fun with details. On landing after our 240,000-mile trip we find that there is not only neither air nor water, but that the Moon is stone cold; even when the Sun shines on it, it is freezing cold, while the temperature drops to perhaps 300 degrees below zero when the Moon’s night comes on. Our next observation will probably be that we feel much lighter than we did on Earth and we are lighter in the very nature of things, for the weight of bodies on the Moon is only one-sixth as much as they are on the Earth. This is due to the Moon being so small and light that gravity has only one-sixth as great an attractive force there as it has on the Earth. Supposing we still retained on the Moon as much strength as we had on the Earth; then every time we took a step we would cover a distance of 15 or 20 feet; if we jumped we would sail through space with the agility of a Harlem goat, and if we played ball and batted the sphere fairly it would shoot off a quarter of a mile and go twice as far from the home plate. [Illustration: FIG. 115.—HOW NEAP TIDES ARE FORMED.] [Illustration: FIG. 116.—HOW NEAP TIDES ARE FORMED.]