6l2 



NATURE 



{Oct. 23, 1! 



rotating by means of a ridge on its inner surface and a 

 corresponding groove on the upright bar. This cylinder 

 carries the writing arm and stylus, which registers on 

 the cylinder the rising and falling of the former ; / is the 

 motive power of the cylinder 111. The apparatus for de- 

 taching the falling weight g is not shown in the diagram, 

 but might be of the following arrangement :— A bob sus- 

 pended by a spiral spring is made to make contact with a 

 cup of mercury, as in the old form of vertical seismo- 

 meters, besides a small lever of the first order attached 

 at one end to the bob, the other extremity being above 

 another cup of mercury. In this way, whether the move- 

 ment of the bob be either up or down, in relation to the 

 mercury cups, contact will be either made in the first case 

 through the lever, or in the second directly by the bob. 

 The current thus established could be used by an electro- 

 magnetic apparatus for removing a catch which holds the 

 weight g. It could also start the cylinder m and stop a 

 clock. The diagram will sufficiently explain the action 

 of the apparatus. 



If we review the advantages and disadvantages of the 

 different instruments, I venture to say that, though far 

 from perfection, they have much to be said in their 

 favour. Their principal feature is the capability of regis- 

 tering continuously all the variations of the earth's move- 

 ment during the complex disturbance known as an earth- 

 quake ; that by employing large tracing drums with a 

 spiral arrangement and time-ruled paper, accurate time- 

 records can be obtained for a considerable period and 

 without interruption, so that a single observer could have 

 under his command a large number of instruments, even 

 at stations some considerable distance apart, thus result- 

 ing in much economy of trained observers. Then again 

 the records are all permanent, being graphically inscribed. 

 The instruments for registering azimuth and amplitude, 

 and capable of doing so with the greatest delicacy and 

 friction, in all cases can be reduced to a minimum, or 

 easily calculated. With regard to the registration of the 

 vertical component of an earth-wave, the old form of 

 spiral spring and bob principle may be excluded from 

 consideration as perfectly unreliable ; and even the im- 

 provements by Messrs. Milne and Ewing, and the 

 ingenious idea of Mr. T. Gray, with its mercury trough 

 compensator, cannot give accurate indication of the 

 characters of a group of earth-waves. Another instrument 

 worthy of trial is the hydrometer vertical-motion seismo- 

 graph of Mr. T. Gray (Phil. Mag., September 1881, 

 p. 209). I think, however, that this instrument might 

 be improved by using a long thin glass tube filled 

 with air and floating in ether or some other fluid 

 of very low viscosity. I would, however, venture to 

 predict that a seismometer based on the principle of a 

 falling weight, being accelerated or retarded according 

 as the earth moves up or down, will supersede other 

 methods, although no doubt such means of registering 

 as described in this paper may be greatly improved upon. 



The instruments described in this paper are all of con- 

 siderable size, but it seems impossible to get good results 

 unless heavy weights and their attendant mechanisms are 

 used so as to reduce friction to a minimum in the con- 

 sideration of results ; for it is certainly a pity to have 

 imperfect results in consequence of limiting the size of 

 the apparatus. One great objection to the falling weight 

 seismometer is the necessity for a deep well, to give suf- 

 ficient time to register an earthquake of ten or twenty or 

 even more seconds' duration ; yet, by giving the weight 

 more work to do by the introduction of multiplying wheels, 

 this might be reduced as the circumstances might demand. 



These instruments and the remarks on them are the 

 outcome of long meditation while wandering over the 

 ruins of two great earthquakes, and although expressed 

 without a technical knowledge of mechanical construction, 

 I hope I have made my ideas sufficiently clear. 



H. J. Johnston-Lavis 



INTERNATIONAL WEIGHTS AND 

 MEASURES 



IN inviting attention to the work done at Sevres during 

 the past year by the Comite - International des Poids 

 et Mesures, 1 we are glad to have the opportunity of con- 

 gratulating those interested in accurate measurement on 

 the fact that this country is now to be represented on the 

 Committee, and will thus have a voice in their discussions. 

 This, as we have pointed out on previous occasions, 

 appears to be required of a country so largely interested 

 in scientific research as ours. 



Metres 



pages. (Paris : Gauthier-Villars, 



