When generalizing the theory of pendulums of Jacob Bernoulli (1654-1705) he discovered a principle of dynamics so simple and general that it reduced the laws of the motions of bodies to that of their equilibrium.
Very long pendulums (30 to 40 ft.) are less subject to this disadvantage, but on the other hand their installation is a matter of some difficulty.
Revolving pendulums are usually constructed with pairs of rods y b and bobs, as OB, Ob, hung at opposite sides of the spindle, that the centrifugal forces exerted at the point 0 may balance A each other.
With pendulums of shorter length, say metres, it is necessary to have a multiplication 80 to too fold by a double system of very light levers, in order to render the extremely slight tilting of their support perceptible.
Attention was first directed to the possibility of rendering ordinary pendulums more truly astatic by Professor Thomas Gray, who suggested methods by which this might be accomplished.
The movements of the apparatus, which when complete should consist of two similar pendulums in planes at right angles to each other, are recorded by means of a beam of light, which, after reflection from the mirror or mirrors, passes through a cylindrical lens and is focussed upon a moving surface of photographic paper.
In Japan, Germany, Austria, England and Russia horizontal pendulums of the von Rebeur-Paschwitz type are employed, which by means of levelling screws are usually adjusted to have a natural period or double swing of from 15 to 30 seconds.
To obtain a complete record of horizontal motion, two of these pendulums are placed at right angles; and by cranking one of the writing levers, o'p', as shown in the plan of fig.
At Rocca di Papa near Rome there is a pair of horizontal pendulums with booms 8 ft.