260 APPENDIX. 



Fig. 18, p. 50, may be very easily made by using a round pasteboard 

 box a few inches in diameter, and securing a piece of lead inside by 

 loops made with a needle and thread. The object shown by Fig. 19 

 may be cut in one piece out of a pine shingle, the centre rod being 

 lengthwise with the grain ; the two extremities are shaved small, and 

 wound with thick sheet-lead, and the whole then colored or painted a 

 dark hue, to render the lead inconspicuous. The experiment with the 

 penknives, p. 51, is very simple, care being taken to insert them low 

 enough in the stick. 



9. Irregular pieces of board, variously perforated with holes, and 

 furnished with loops to hang on a pin, may be used to determine the 

 centre of gravity, according to the principle explained by Fig. 21, p. 51. 



10. Portions of plank and blocks of wood, with the centre of gravity 

 determined as in the last experiment, may have a plumb-line (which 

 may be a thread and small perforated coin) attached to this centre, and 

 then be placed on differently inclined surfaces, to show their upsetting 

 just as this line of direction falls without the base. Toy-wagons, 

 bought at the toy-shops, may be variously loaded and used in experi- 

 ments of this sort. 



1 1 . Experiments with the lever of the first kind may be easily per- 

 formed by the use of a flat wooden bar, two or three feet in length, 

 marked into inches, and placed on a small three-cornered block as a 

 fulcrum. Weights, such as are used for scales, may be variously 

 placed upon the lever. Levers of the second and third kind, which 

 are lifted instead of borne down, may have a cord attached to the point 

 where the power is to be applied, running up over a pulley or wheel, 

 with a weight suspended to the other end. 



12. An axle, furnished with wooden wheels with grooved edges, of 

 different sizes, may be used to exhibit the principle of the wheel and 

 axle, in connection with scale-weights that are furnished with hooks. 

 The power of combined cog-wheels may be shown by a combination 

 like that represented on p. 76, using weights for both cords. 



13. Interesting experiments with the inclined plane, at different de- 

 grees of slope, by a contrivance similar to that represented by Fig. 87, 

 p. 104, with the addition of a small wheel at the upper side for a cord 

 to pass over. This cord is fastened at one end to a light toy-wagon, 

 running up and down the plane, and at the other to a weight suspend- 

 ed perpendicularly just beyond the upper edge of the plane. The 

 wagon is variously loaded with weights to counterpoise the suspended 

 weight at different degrees of inclination. 



