March 13, 1884] 



NATURE 



457 



daily observation. At Greenwich Observatory the tremu- 

 lous motion in the soil, especially noticeable on bank 

 holidays and at times when Greenwich Park was un- 

 usually crowded, resulted in the construction of an 

 apparatus in which the dish of mercury used in the 

 determination of the collimation error of the transit circle 

 was suspended by flaccid springs. By means of this con- 

 trivance the tremulous motions of the ground were ab- 

 sorbed before they reached the mercury, and the difficulty 

 of observation was overcome. French engineers, working 

 with delicate surveying instruments in crowded cities, 

 have similarly been compelled to suspend a portion of 

 their apparatus, so that a steady image could be obtained. 

 Prof H. M. Paul, seeking for a site for the Naval Obser- 

 vatory at Washington, found that the image of a star re- 

 flected from a tray of mercury was disturbed by a train 

 passing at the distance of a mile. Lieut. -Col. Palmer, 

 when engaged in observing the transit of Venus in New 

 Zealand, discovered that a ditch a few feet in depth was 

 sufficient to intrench his instruments against the disturb- 

 •ance created by trains passing at a distance of 700 yards. 

 Capt. Denman found the effect of a goods train to be 

 transmitted 1 100 feet over marshy ground, but vertically 

 above the train, passing through a tunnel in sandstone, 

 the disturbance extended only 100 feet. One result ob- 

 tained from these and numerous other observations upon 

 artificially produced tremors indicates that these disturb- 

 ances are superficial, and although they may creep up 

 the surface of a gently sloping hill, their spread is checked 

 by a steep cutting. 



Naturally produced tremors differ from those just 

 spoken of by the fact that their distribution is not so 

 superficial, and not only are they to be observed in the 

 most substantial structures which engineers can design, but 

 they are to be equally well seen in cellars and in the walls 

 of rocky caves. Some knowledge of the depth to which 

 they extend might be obtained by a few microseismic ob- 

 servations in the deep mines of Lancashire and other 

 parts of the United Kingdom. As the observations are 

 so simple, and the instrument required so easily con- 

 structed — in fact, it may be home made — it is earnestly 

 desired that some of our mine managers will spontaneously 

 undertake this work. 



I make this suggestion, not only on account of the 

 scientific value of the work, but because there are reasons 

 10 believe that such observations may lead to results of a 

 practical value by relations they may ho'd to the escape 

 of gas, the circulation of subterranean waters, and other 

 underground phenomena. The instrument I should re- 

 commend for this purpose is the tromometer of Bertelli 

 and Rossi. This is shown in the accompanying figure, 

 r. is the bob of a pendulum about 100 grammes in weight, 

 suspended by a very fine wire about lA metres in length. 

 The whole is inclosed in a tube. The style s of this pen- 

 dulum is seen reflected by the prism P by means of the 

 microscope M. The eye-piece e of this microscope con- 

 tains a micrometer scale, Idv which to measure the ampli- 

 tude of the motion of the style. 



The direction of motion may be obtained by turning 

 the eye-piece until the scale is parallel with the direction 

 of motion, and this direction then read off from the posi- 

 tion of an index moving over compass divisions marked 

 on the fixed tube of the microscope. To commence with, 

 the style of a pendulum might be looked at directly with 

 a microscope, or two microscopes placed at right angles, 

 having magnifications of forty or fifty diameters; and if 

 it was found that movements existed, the prism and micro- 

 meter scale might be added subsequently. The pendu- 

 lums may be hung from spikes driven in the solid rock or 

 from an iron stand. 



The chief results which have been obtained with instru- 

 ments of this type are those which have been arrived at 

 in Italy. The father of the science of microseismology 

 is Father Bertelli of Florence, who, since 1870, has made 



many thousands of observations under a variety of cir- 

 cumstances. Another ardent worker at this subject is 

 Prof. M. S. de Rossi at Rome, by whose exertions 

 numerous observatories have been established throughout 

 the whole of Italy where these observations are systemati- 

 cally carried on. In making these observations every 

 precaution appears to have been taken to avoid accidental 

 disturbances, and the experiments have been repeated in 

 a variety of forms. The results which from time to time 

 have been announced are of the greatest interest to those 

 who study the " physics of the earth's crust," and appear 

 to be leading not only to the establishment of laws of 

 scientific value, but also to the elucidation of phenomena 

 which have an intimate connection with our every-day 

 existence. 



It would seem that the soil of Italy is in incessant 

 movement, there being periods of excessive activity usually 

 lasting about ten days. Such a period may be called a 

 seismic storm. These storms are separated by periods of 

 relative calms. The storms have their greater regularity 



in winter, and sharp maximums are to be observed in 

 spring and autumn. In the midst of such a period, or at 

 its end, there is usually an earthquake. LIsually these 

 storms are closely related to barometric depressions. To 

 distinguish these movements from those which occur 

 under high pressure, they are called baro-scisinic move- 

 ments, the latter being called volcano-siismic movements. 

 The relation of these storms to barometric fluctuation 

 has been observed to be very marked during the time 

 of a volcanic eruption. At the commencement of a 

 storm the motions are usually small, and one storm lasting 

 two or three days may be joined to another storm. In 

 such a case the action may be a local one. It has been 

 observed that a barometrical depression tended to bring 

 a storm to a maximum, whilst an increase of pressure 

 would cause it to disappear. Sometimes these actions 

 are purely local, but at other times they may affect a con- 

 siderable tract of land. 



If a number of pendulums of different lengths are ob- 

 served at the same place, there is a general similarity in 

 their movements, but it is also evident that the free period 

 of the pendulum more or less disturbs the character of 

 the record. The greatest amplitude of motion in a set 

 of pendulums is not reached simultaneously by all the 



