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XIIT. On the Theory and Construction of a Seismometer, or Instrument for Measur- 

 ing Earthquake Shocks, and other Concussions. By JAMES D. FOHBES, Esq. F.R.S. 

 Sec. R. S. Ed., Professor of Natural Philosophy in the University of Edinburgh. 



(Read 19th April 1841.) 



HAVING been requested to act on a Committee of the British Association, 

 appointed to devise and apply methods for measuring the comparative intensity 

 of earthquake shocks, and having been shewn several ingenious contrivances by 

 Mr DAVID MILNE (who suggested the inquiry) Lord GREENOCK, and other per- 

 sons, an apparatus occurred to me which should unite the requisites of Simpli- 

 city, Compactness, Comparability, and an easy adjustment of Sensibility accord- 

 ing to circumstances. 



Mr MILNE had not failed to distinguish the ends for which instruments (which 

 for obvious reasons were to be self-registering) ought to be devised, such as the 

 measurement of horizontal concussions, of vertical elevation, and of heaving or 

 angular motion of the surface. It is no part of my present object to consider the 

 probable movements of the soil in earthquakes. I limit myself to the description 

 of a single instrument intended to measure lateral shocks, such as are expe- 

 rienced by objects placed upon a table which is abruptly shoved forwards. 



A heavy pendulum suspended from a frame in such a manner that the iner- 

 tia of the bob should cause it to oscillate when its centre of suspension had been 

 displaced by the movement of the frame with which it was connected, had already 

 been suggested for the purpose. To obtain sufficient sensibility, a pendulum of 

 great length would be required, nor could the sensibility be altered according to 

 circumstances, being wholly independent of the weight of the bob. The unwieldi- 

 ness of a pendulum ten or twenty feet long alone forms a strong objection to this 

 apparatus. 



The elegant inverted Pendulum or Noddy contrived by the late Mr HARDY,* 

 suggested to me a different arrangement. The instrument is seen in Elevation, 

 Section, and Plan, in Plate III. Figures 1, 2, and 3. A vertical metal-rod A B, 

 having a ball of lead C moveable upon it, is supported upon a cylindrical steel- wire 

 D, which is capable of being made more or less stiff by pinching it at a shorter or 



* Described by Captain Kater, Phil. Trans. 1818. 

 VOL. XV. PART I. 3 N 



