Fig. 70. Hygrometer
Next morning Fred asked his father to enlighten himself and George regarding the making of a few simple meters, such as barometer, hygrometer, and a thermometer. He also wished to know if it would be possible for him to make a boomerang. Mr. Gregg told him he would be pleased to help him, and that there would be no difficulty in making a boomerang if he went to work at it earnestly.
On the arrival of his father that evening, the subject was again introduced, and Mr. Gregg using the blackboard, laid out the following drawing and wrote the accompanying instructions.
"The best hygrometer of absorption is (according to Deschanel) that of De Saussure, consisting of a hair deprived of grease, which by its contractions moves a needle. When the hair relaxes, the needle is caused to move in the opposite direction by a weight which serves to keep the hair always tight as seen in the illustration, [Fig. 70]. The hair contracts as the humidity increases. In the accompanying illustration A A and B B represent the frame; e f, the scale; a, screw for tightening the hair; b, the hair; O, weight; H, thermometer.
Fig. 71. Index of Hygrometer
"A neater hygrometer, and one on the same principle, may be made by taking an old tooth powder box (as deep a one as possible, since the longer the string, the more sensitive it is), and boring a hole through the centre of the top and bottom. Paste a kind of dial in paper on the top of the box; take a piece of catgut, or small fiddle string, and push it up through the hole in the bottom and out at the one in the lid. Glue the bottom end immovably, and let the top end move freely: make a small index of a strip of whalebone ([Fig. 71]); bore a hole in the centre, and fix it on the catgut with glue. Wet the catgut, see which way it turns, and mark 'wet' and 'dry', accordingly on the dial.
"So much for the hygrometer. Now about that curious thing, the boomerang. If the following directions are closely adhered to, and the proper shape followed, a regular Australian boomerang will result. It is not difficult to make. Take a piece of hard wood, the natural shape of one of the segments of an ordinary wheel felloe, or bend in the wood; let it be 1⁄4 inch thick, shaped as at [Fig. 72], to be held in the right hand at A, which shows the way the edges of the side facing the left hand must be bevelled off. It requires a slight curve on the flat side; so that, if on a table, each end would turn about 1⁄8 inch. It is then a part of a very fine pitch screw, in motion similar to a piece of slate jerked into the air, the sole difference being due to the slight curve in the back, which gives the screw motion, in conjunction with the forward and rotatory motion given by the hand. Sheet-iron would not do, as there would not be thickness to show the bevelled edge. The boomerang was made in the form of a cross, with four legs of equal length, bevelled, but it does not work as well as the regular form. You must be careful in throwing it as it may strike you on return."