=To Light a Fire with the Heat of the Sun.=—A small _magnifying glass_, or burning glass, is simply a convex lens. It is a little piece of apparatus that every boy should always carry with him just as he does his pocket knife and compass. A lens 1½ inches in diameter and having a 4-inch focus may be bought for 25 or 30 cents. A lens of this kind will be found very useful in many ways, for it will greatly magnify any object such as cloth, leaves, insects, finger-prints, in fact anything you may wish to see better than you could with your naked eye, though you cannot use a single lens for a spyglass. A magnifying glass will also frequently come in handy for lighting fires, by using the Sun’s rays when matches are scarce. While a Boy Scout would disdain to use paper to kindle a fire, yet if a scrap of paper is at hand it will prove a good medium on which to direct the rays of the Sun with a burning glass. If you have no paper focus your glass on some _punk_ or very dry leaves, as shown in Fig. 37. To focus the glass means to hold it away from the paper or leaves so that the rays of the Sun are brought to a point like the sharpened end of a lead pencil; when all the rays of sunlight, each of which carries a little heat, are brought to a point, they will make enough heat to light a piece of paper or a dry leaf. [Illustration: FIG. 37.—BOY FOCUSING BURNING GLASS ON LEAVES TO MAKE FIRE.] =Signaling with the Sun’s Rays.=—There are many ways of sending a signal or a message across space by day, as, for instance, by means of smoke, by flags and flashes of sunlight; by bonfires, pine-knot flames and burning arrows by night, and by wireless, which can be used either by day or by night. A simple and effective way to signal in the daytime when the Sun is shining is by using a mirror, that is, a looking-glass, as it is commonly called. Every boy knows how to make flashes with a mirror, so it will be enough to say here that the glass is held in the hand in such a position that the sunlight falling upon it will be reflected in the direction you wish to send the signals. Fig. 38 shows how it is done. [Illustration: FIG. 38.—BOY SENDING FLASH SIGNAL WITH MIRROR.] [Illustration: FIG. 39.—CONTINENTAL MORSE CODE.] Any sort of a code can be used, but it is far more interesting and will prove very useful if you are able to send and receive messages in the dot and dash alphabet, or Morse telegraph code, which is given in Fig. 39. A short flash represents a dot, a long flash a dash and short and long flashes represent letters. This is the same code that is used for wireless telegraphy. =How to Make a Simple Heliograph.=—A _heliograph_ is merely a mirror mounted on a baseboard, but this is a big improvement over holding the mirror in the hand, for to send and receive flashes over long distances the mirror must be carefully aimed and kept in position.