581. You can draw on rough paper with charcoal.

582. When little children get new shoes, the soles should be scratched and made rough.

583. You can get your face very clean by rubbing cold cream into it, then wiping the cold cream off on a towel or cloth.

584. Soft paper blurs writing when you use ink.

585. Water will flow over the side of a pan through a siphon, if the outer end of the siphon is lower than the surface of the water in the pan.

586. There is a loud noise when a gun is fired.

587. Colored cloths should be matched in daylight, not in artificial light.

588. Lamp chimneys are made of _thin_ gla.s.s.

589. When you sweep oiled floors, no dust flies around the room.

590. The ocean is salty, while lakes are usually fresh.

591. A gla.s.s gauge on the side of a water tank shows how high the Water in the tank is.

592. You burn your hand when you touch a hot stove.

593. Pounding a piece of steel held horizontally over the earth and pointing north and south will make it become a magnet.

594. When only one side of a sponge is in water, the sponge gradually gets soft all over.

595. If we breathe on a cold mirror, a fine mist collects on it.

596. b.u.t.ter is kept in cool places.

597. Water will boil more quickly in a covered pan than in an open one.

598. Mucilage, glue, and paste all become hard and dry after being spread out on a surface for a while.

599. You cannot see things clearly through a dusty window.

600. In making fire grates it is necessary to have the bars free to move a little.

APPENDIX

A. THE ELECTRICAL APPARATUS

For giving children a practical understanding of such laws of electricity as affect everybody, the following simple apparatus is invaluable. It is the electrical apparatus referred to several times in the text. The only part of it that is at all difficult to get is the nichrome resistance wire. There is a monopoly on this and each licensee has to agree not to sell it. It can be bought direct from the manufacturer by the school board if a statement accompanies the order to the effect that it is not to be used in any commercial devices, nor to be sold, but is for laboratory experimentation only. The manufacturers are Hoskins Manufacturing Company, Detroit, Michigan.

The following diagram will make the connections and parts of the electrical apparatus clear:

[Ill.u.s.tration: FIG. 190. Electrical apparatus: At the right are the incoming wires. Dotted lines show outlines of fuse block. _A_, 2 cartridge fuses, 15 A; _B_, 2 plug fuses, 10 A; _C_, knife switch; _D_, fuse gap; _E_, snap switch; _F_, _H_, lamp sockets; _G_, flush switch; _I_, _J_, _K_, nichrome resistance wire, No. 24 (total length of loop, 6 feet), pa.s.sing around porcelain posts at left.]

The flush switch (G) should be open at the bottom for inspection,--remove the back. The snap switch (E) should have cover removed so that pupils can see exactly how it works.

The fuse gap (D) consists either of two parts of an old knife switch, the knife removed, or of two bra.s.s binding posts. Across it a piece of 4-ampere fuse wire is always kept as a protection to the more expensive plug and cartridge fuses. Between the resistance wire (_I_, _J_, _K_) and the wall should be either slate or sheet asbestos, double thickness. Under the fuse gap the table should be protected by galvanized iron so that the melted bits of fuse wire can set nothing on fire when the fuse wire burns out.

B. CONSTRUCTION OF THE CIGAR-BOX TELEGRAPH

The "cigar-box telegraph" shown on page 381 is made as follows: An iron machine bolt (A) is wound with about three layers of No. 24 insulated copper magnet wire, the two ends of the wire (_B_, _B_) projecting. The threaded end of the bolt (C) is not wound. A nut (D) is screwed on the bolt as far down as the wire wrapping. The threaded end is then pushed up through the hole in the top of the cigar box as that stands on its edge. Another nut (E) is then screwed on to the bolt, holding it in position. The bolt can now be raised or lowered and tightened firmly in position by adjusting the two nuts (_D_ and _E_), one above and one below the wood.

A screw eye (F), large enough to form a rest for the head of another machine bolt (G), is screwed into the back of the box about three fourths of an inch below the head of the suspended bolt (A). Two or three inches away, at a slightly higher level, another screw eye (H) is screwed into the back of the cigar box. This screw eye must have an opening large enough to permit an iron machine bolt (G) to pa.s.s through it easily. A nut (I) is screwed down on the threaded end of a machine bolt until about an inch of the bolt projects beyond the nut.

This projecting part of the bolt is then pa.s.sed through the screw eye (H) and another nut (J) screwed on to it to hold it in place. This nut must not be so tight as to prevent the free play of the bolt as its head rises and falls under the influence of the vertical bolt.

The head of the horizontal bolt rests upon the screw eye which is immediately below the head of the suspended bolt. You therefore have the wrapped bolt hanging vertically from the top of the box, with its head just over the head of the horizontal bolt. There should be about one quarter inch of s.p.a.ce between the heads of the two bolts. An electric current pa.s.sing through the wires of the vertical bolt will therefore lift the head of the horizontal bolt, which will drop back on to the screw eye when the circuit is broken.

[Ill.u.s.tration: Fig. 191. The cigar-box telegraph.]

COMMON SCIENCE

NEW-WORLD SCIENCE SERIES _Edited by John W. Ritchie_

SCIENCE FOR BEGINNERS By _Delos Fall_

TREES, STARS, AND BIRDS By _Edwin Lincoln Moseley_

COMMON SCIENCE By _Carleton W. Washburne_

HUMAN PHYSIOLOGY By _John W. Ritchie_

SANITATION AND PHYSIOLOGY By _John W. Ritchie_

LABORATORY MANUAL FOR USE WITH "HUMAN PHYSIOLOGY"

By _Carl Hartman_

EXERCISE AND REVIEW BOOK IN BIOLOGY By _J. G. Blaisdell_

PERSONAL HYGIENE AND HOME NURSING By _Louisa C. Lippitt_

SCIENCE OF PLANT LIFE By _Edgar Nelson Transeau_