The [7th. Figure] exhibits a spring adapted to heavier purposes. It is a cylinder nicely bored and hermetically closed at bottom; in which works a Piston P plunged in oil, which when forcibly drawn up forms a vacuum in the cylinder, into which the atmosphere endeavouring to enter, acts like a spring on the Piston; and preserves the same stress whatever be the height of this Piston in the cylinder.
This then, is also an equalized Spring, such as these experiments require; but it is not my invention. I first saw a vacuum used, as a spring, by my noble Patron, the late Earl Stanhope: to whose mechanical attainments, I owe this tribute of applause on the present occasion.
In the three Figures of this [Plate], [8], [9, 10], are shewn two of the means I use for creating those factitious resistances that are sometimes wanted in the process of measuring power. In [Fig. 8], E H F, is a gripe or brake, such as millers use to stop their wind-mills with; fixed under L, it surrounds the wheel E H, and is then fastened to the end F of the lever K L. The brake is thus pressed with greater or less force against the wheel, as the weight I is placed more or less distant from the fulcrum L of the lever. By these means a resistance of the equable kind is produced, capable of being adapted to any power it may be wished to measure; which makes this Dynamometer a real tribometer or measurer of friction.
The second kind of resistance brought forward in this Plate, is a Pendulum P ([Fig. 9 and 10],) set a vibrating by a pallet-wheel A B, connected with the axis of resistance; and working in the pallets N. It appears besides, in the Figure, that the times of vibration can be changed by the mechanism T N R, which raises or lowers the ball P. This then, is another resistance, such as we sometimes want: but it is also a mean of finding the quantity of resistance that a vibrating body opposes to motion, when oscillating in times not those due to its length as a pendulum. In other words it is a mean of measuring vis inertiæ itself—which an astounding modern writer declares does not exist!