GEOMETRY, DCSCript Many devices have been invented by which (Engrain, illustrating natural phenomena may be automatically described. The physical ex periment which produces the parallelogram of forces is a familiar example. As a more general illustration. suppose a spring dynamometer (9.v.) placed where it may receive the draught of a horse when moving a carriage. Let the move ment of the spring shown by an index whose motion is back and forth along a line in the direct ion of the draught. Fix a peneil to the index, and let it, point rest upon a sheet of on a plane or a cylinder %%11101 moves at a Uniform velocity in a direction p•rpendieular to the mo of the nob \. The combined movements of the pencil and paper beneath it will trace a more or le-, irregular line. If the force of Irauglu is constant, the pencil will remain at a constant distance from the edge of the paper, and the trace will be parallel to the edge. If the paper move while the pencil varies, the line will be perpendicular to the edge. It both move, is to a diminishing force, the trace will be oblique, approaching one edge: tine If the force increases, the oblique trace will diverge from the -ante edge. Snell mechanism is often arranged fur instruments which indicate meteorological. physiological, or other changes,
a- the force direction of wind-, or the pulsa tion of the arteries (see SPIIYCMoGRAI.II), or the Movements of a clock combined with the observa tions of an astronomer. (See Cams:Nth:itself.) An important application of the same principle is found in the indicator diagram, by which the pressure of the -team in the steam-engine and the work done by each stroke of the piston be come matter: of record. The paper moves with the movement of the piton. both in its excursion and return: the pencil move- at rio.ht angles to the direction of the motion of the paper, under the influence of the steam pressure. and the dia gram drawn shows for each instant of the stroke the volume and pressure of the steam. while the total area of the diagram indicates the amount of work done. A mechanical device to illustrate the luminosity of various colors was invented by Ntiwton, and is known as Yirton's diagram. In mechanics the stresses on the various parts of a frame or bridge are graphically exhibited by a form known as a stress or funicular dia gram. The seismometer, seismograph, or Se jSMO SrOpe is an instrument to record the velocity, di rection, and magnitude of seismic waves. See