In this paper the torsion seismograph is considered comprehensively. Reference is made to the research program, under which it was developed, and to several cognate undertakings. The advantages of photographic registration, and of small instrumental dimensions, are pointed out. The principle of the torsion suspension, and its properties, are discussed. The construction of the instrument and its adjustments are described. An abridged theory of the seismometer is given, especially for the case of critical damping. Its action as an accelerometer is emphasized. The terms “efficiency,” “magnification,” and “efficiency with respect to velocity of vibration,” and also “sensitiveness” are formulated and defined. Methods for determining the constants are given. Auxiliary apparatus and methods are described.

Appendix: In the appendix the theory of the seismometer is treated more fully and more generally. Special cases are discussed. “Sensitiveness” is discussed in its relation to “efficiency” and “magnification,” and to damping also. The decay of proper motion is considered and the best value of damping is approximated. The case of an “impulse” is defined and considered; also the case of a number of successive “impulses,” as defined. The case of a constant angular acceleration is discussed. Finally, sample seismographic records are shown and described.

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