PARTRIDGE & BRITTAN, Publishers,
No. 300 Broadway, New York.
FOOTNOTES:
[1] See subsequent investigation on Media; also [Plate IV]. and description.
[2] These disks cost at the foundry about 37½ cents a piece. One may be used as a pattern by which to cast others.
[3] There is also this difference, that in Fig. 2 the board is supported by only three wheels, so as to have one in front under the hands of the medium, by which sufficient pressure is secured to make its rotation certain. But as the position thus given does not fall into the plane of the pulley at the back of the disk, the wheel in question is supported upon an axle which is secured in staples or holes, and carries a pulley just at the position where it is coincident with the plane aforesaid. The wheel is visible in front.
[4] Vis inertiæ, or force of inertness, is the force by which a body, when at rest, resists being put into motion, or, when in motion, resists arrestation. The force, in this latter case, is called momentum, being directly as the weight multiplied by the velocity. Thus, two pounds, moving at the rate of one foot per second, exercise exactly the same momentum as one pound moving at the rate of two feet per second.
The force of a spring, or of explosive compounds, cannot be called momentum; neither velocity nor weight enter into its constitution; though, when transferred to a projectile, it produces momentum proportional to the force with which it acts, the weight moved, and velocity imparted.
Muscular force does not come within the definition of momentum, although it produces this property in a hammer, proportionably to its weight and the resulting velocity. Nor is the force of gravity momentum, though momentum be generated by it in falling bodies.
[5] It is suggested that these words may be misapprehended. I use them in the sense given by Johnson: “Sight of any thing, commonly mental view.”