The literary digest . moon with a pencil. Mercury and Venus come in as dots of intermediate size,spaced out somewhere between the earth and the sun, or im-agined in some of the near-by rooms. Mars, a dot the size ofthe periods in the text-book, chances most conveniently tospace out just at the end of the room diagonal. For the major planets, the class has to look out the windowand imagine dots of the proper size marked in the proper placeon various near-by objects. Practically, I take a map of theregion and strike circles. Jupiter is, of course, about the sizeof a blackboard crayon and 160 fee
The literary digest . moon with a pencil. Mercury and Venus come in as dots of intermediate size,spaced out somewhere between the earth and the sun, or im-agined in some of the near-by rooms. Mars, a dot the size ofthe periods in the text-book, chances most conveniently tospace out just at the end of the room diagonal. For the major planets, the class has to look out the windowand imagine dots of the proper size marked in the proper placeon various near-by objects. Practically, I take a map of theregion and strike circles. Jupiter is, of course, about the sizeof a blackboard crayon and 160 feet away. Neptune, not muchmore in diameter than half the thickness of the centimeter rule,is nearly a fifth of a mile off When the class reach comets, I remind them of the modelalready developed. Then we draw in a few specimens, from alittle chap only a yard long with a head the size of the earthup to such a visitant as fills the whole side of the class-room. By and by, we strike double stars. Algol, as the type of the. Conrtciy vt IV-pular iMecluiuics. Cliira^o. A BOON FOR AVIATORS Tlie blow of tlie monkey-wrench meets a12,000 pounds per squart spectroscopic binary, works out as two bodies, one the size of themodeled sun, the other a little larger, which are 30 centimetersapart, and revolve round one another in three days. The tele-scopic binaries, on the other hand, have orbits of the same gen-eral order as Uranus and Neptune. They are, then, easily im-agined on the grounds of some other school Scaled off on our model, the nearest fixt star is fairly repre-sented, let us say, by the usual globe of an arc light, set 1,600miles away. In other words, we imagine Alpha Centauri asone of the street-lights of Denver. With this terrestrial baU but 8,000 miles in diameter, onlynineteen known stars, Mr. Brewster finds, can go on the earthat all. He continues, therefore, by placing two or three objects—say, a marble, a watermelon, and a circus balloon—in Petrograd,Valparaiso, and Hon
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