116 



Fig. 126. 



ball, representing the moon, which is movable nearly around 

 the globe ; brass wires proceed from the sun, representing 

 the rays of the sun's light falling on the side of the earth 

 turned towards the sun. From this end of the instrument 

 various eclipses may be shown. From the other side of the 

 sun there is a ball representing the moon, by which the 

 earth is eclipsed, and wires representing rays from the sun, 

 for explaining the shadows called penumbra and umbra, 

 and is a valuable instrument in explaining both solar and 

 lunar eclipses. Price, 85.00. 



The Tide Dial. (Fig. 127, next page.) This instrument 

 consists of a circular piece of wood, on which there is re- 

 presented a dial having the twenty-four hours of the day ; 

 connected at the centre, to an axis, is a movable disk repre- 

 senting the ocean, and bearing an arm having a ball attached 

 to the end, representing the moon ; the water appearing at 

 high tide in that direction ; the outer disk representing the 

 earth. On the back of the dial is a crank, giving motion to 

 the wheel-work, and causing the earth and moon to revolve ; 

 the earth revolving twenty-nine times faster than the moon. 



The tides are occasioned by the ebbing and flowing of the 

 sea, which are caused by the attraction of the sun and 

 moon, but chiefly by the latter. 



The attraction causes the water to assume a spheroidal 

 figure, the longest axis being in the direction of the moon. 



This oval of the waters keeps pace with the moon in its 

 monthly course round the earth ; while the earth, by its 

 daily rotation on its axis, presents each part of its surface to 

 the direct action of the moon, twice every day, and thus 

 produces two flood and two ebb tides. But because the 

 moon is in the mean time passing from east to west in its 

 orbit, it comes to the meridian of any place later than it 

 did the preceding day ; consequently the two floods and 

 ebbs require nearly twenty-five hours to complete them. 



The tides are greatest at the new and full moons, and are 



