282 APPENDIX. 



dark hue, to render tlie lead inconspicuous. The experiment witli the 

 penl'Cnives, p. 35, is very simple, care being taken to insert them low 

 euougli in the stick. 



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

 nished with looj^s to hang on a pin, may be used to determine the centre 

 of gravity, according to the principle explained by fig. 21, p. 35. 



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. To3--wagous, bought 

 at the toyshops, may be variously loaded and used in experiments of 

 tliis sort. 



11. 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 i:)laced on a small tliree-cornered block as a 

 fulcrum. Weights, such as are used for scales, may be variousl}'' 

 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 i:ower is to be applied, running up over a pulley or wheel, 

 with a weight suspended to the other end. 



13. An axle, furnished with wooden wheels witli grooved edges, of 

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

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

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

 like that represented on p. 57, 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. 96, 

 p. 83, Avith 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, run- 

 ning up and down the plane, and at the other to a weight suspended 

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

 is variously loaded with weights, to counterpoise the suspended weight 

 at difierent degrees of inclination. 



14. A lecturer may quickly demonstrate before a class the small in- 

 crease in the length of a road, in consequence of a considerable curve 

 to one side of a straight line (as shown by fig. 09), by using a cord for 

 measuring, the diagram being marked on a board or the wall. 



15. A round stick-of wood, and a long, wedge-shaped slip of paper, 

 easily show the principle of fig. 75, i). 70. 



16. A cog-wheel with endless screw and winch (fig. 77, p. 71), exhibits 

 distinctly the great i)ower of the screw in this combination. 



17. Tine sticks, two feet long, and one-fourth to one-half inch through, 

 of different shapes and sizes, sui)ported at each end, and with weights 

 hung at the middle till they break, may be made to illustrate the princi- 

 ples described on pp. 80, 81. 



18. Some of the principles of draught may be shown, and especially 



