254 
THE TEACHING OE MECHANICS BY EXPERIMENT. 
for use. The same feature will be noticed in most of the other pieces 
of apparatus I have to bring before you. 
Passing now to experiments by help of which the principles 
of statics may be exemplified quantitatively, we get quantitative 
examples of the composition of forces in an apparatus in which 
three cords are attached to a small ring, one of the ends being directly 
loaded by weights, while the other two are loaded by weights on cords 
which pass over pulleys. When a triangle is sketched with its three 
sides parallel to the directions taken by the three cords with the weights 
in equilibrium, the lengths of its three sides will be proportional to 
the three forces. The apparatus is mounted on a vertical board cov¬ 
ered with squared paper. The co-ordinates along each cord between 
any two points are read off and in that way the tangent of the angle 
of inclination of the cord is determined. Having observed these in¬ 
clinations, the student transfers the directions of the lines to his draw¬ 
ing board'and he then proceeds to draw the triangle, to measure its sides, 
and to compare them with the known values of the forces. He finds 
that there is some uncertainty on account of the friction of the pulleys, 
and that the ring to which the cords are attached may be made to 
take up its position anywhere within a certain limited range, and by 
examining the possible range he finds the place which it might be 
expected to stand in if there were no friction. Another experiment of 
the same kind is the funicular polygon, formed by mounting a cord on 
a similar board and hanging loads from it at certain fixed points. 
The form of the polygon is again determined by help of the squared 
paper behind, and is transferred to the drawing board, after which the 
diagram of forces is drawn and the segments of the load-line are 
measured and compared with the known weights. 
The inclined plane is another simple piece of apparatus giving ex¬ 
periments in the resolution of forces. A sloping board forms the 
plane and a block rests on it with pieces of wire between to serve as 
rollers. This nearly does away with friction between the block and 
the plane. A weight is then applied to the end of a cord which passes 
over a pulley at the top of the plane, and is adjusted until any small 
increase of it causes the block to rise and any small decrease of it 
causes the block to descend. The component of force along the plane, 
measured in this way, is compared with the value calculated from the 
observed inclination of the plane. By leaving out the rollers the 
student can use the same apparatus to make experiments on the coeffi¬ 
cient of friction between the block and the plane. 
To investigate the friction between a cord and a drum or pulley, 
a convenient and simple apparatus, Fig. 2, is formed by clamping a 
pulley against a board covered with squared paper, supporting the 
board vertically, and putting a cord over the pulley. One end of 
the cord carries a hanging weight, the other end passes over a loose 
guide pulley which can be attached at any part of the edge of the 
board, and carries another weight. Either weight may be raised 
until the cord slips over the clamped pulley, and the angle of 
contact may be altered at will. The magnitude of the angle is found 
