ON SEISMOLOGICAL INVESTIGATION. 271 



To extend our knowledge on this subject a central station might be 

 provided with a number of instruments having different periodic motion, 

 luasmucli as very long pendulums, as at Catania, shorter pendulums or 

 horizontal pendulums with a high multiplication, as at Padua and Strass- 

 burg, are affected by wind and other activities disturbing their supports, 

 which at the latter place result in what appears to be continuous 

 movement, it is likely, and in some instances it is certain, that the 

 preliminary tremors of an earthquake have been eclipsed, and its com- 

 mencement therefore been rendered uncertain. 



The general direction in which motion has advanced is known from 

 the time records obtained at several stations. The varying directions at 

 which the ground has moved at a given station may from an instrument 

 recording movements as two rectangular components be sometimes 

 determinable. Usually, however, we are left to choose between two 

 directions. 



The records from Dr. Ehlert's pendulum and the apparatus of Vicentini 

 remove such doubts. 



As to whether a seismogram can or cannot be analysed with regard to 

 the period and amplitude of separate waves simply depends upon the 

 speed at which the record- receiving surface is moved. 



Contrasting the various types of instruments last referred to with the 

 type of instrument adopted by this committee, considering the object in 

 view, there does not appear to be any necessity to regret the choice whicli 

 they have made. 



Each instrument has its merits, and for particular purposes may be 

 better than any other. It was impossible for the committee to have 

 adopted either the long pendulums or large horizontal pendulum, of Italy 

 on account of the difficulty of their installation. The Strassburg pendulums, 

 although most desirable at a central station, require a too carefully 

 insulated installation, and entail too much expense for photographic 

 materials to put them within the reach of ordinary observers. 



Like the Ehlert pendulums, another instrument equally desirable at a 

 central station is the Vicentini pendulum. However, as this requires 

 the addition of a chronometer and delicate manipulation to insure simi- 

 larity in adjustment, and a somewhat high and solid supporting wall or 

 pier, it is likely that private observers might find difficulty in its adoption. 



For further information respecting the various types of seismographs 

 here mentioned tlie reader is referred to the British Association Report 

 for 1896, p. 182. 



From the preceding notes it is clear that in Italy and Germany 

 seism ological investigation receives substantial recognition. 



For many years past in the former of these countries observatories 

 have been established, at each of which we find a resident observer, his 

 assistant and custodian with their necessary dwellings, offices, and work- 

 rooms. When first established the object of these institutions was to 

 record and study the more or less violent movements of the earthy crust 

 which can be felt. To this was added the observation of tlie ubiquitous 

 so-called earth tremors, and partly, perhaps, because it Avas found that 

 these latter in particular were closely associated with certain meteoro- 

 logical conditions, the system was incorporated with the Meteorological 

 Bureau. 



During the last few years tlie observations have been extended to 



