2IO 



NATURE 



[December 27, 1894 



The case is remarkable in more (han one respect. Displace- 

 ments of the light-point, which, though the oscillations of the 

 pendulum were much larger generally, were scarcely noticeable 

 daring the former observations with this instrument at other 

 places, often occur at Strassburg. I am inclined to think that 

 they are due to a vibratory motion of the ground, which scarcely 

 affects the motion of the pendulum, but may cause a change in 

 its position with regard to the sletl pivots. These vibrations 

 appear to be more easily propagated by the soil at Strassburg 

 than at Nicolaiew, for though small displacements occasionally 

 occur at the latter place, they are considerably smaller. This is 

 particularly evident in the present case, where the only displace- 

 ment worth mentioning is connected with the shock at 5'77- O" 

 the other side, the displacement at Strassburg, which produced 

 the long break in the curve, is far the largest that occurred 

 during one and a half years' observation. It is much larger 

 than that which tork place when an iron hook was driven into 

 the pillar on the side opposite to the pendulum. 



Our figure shows that the displacements of the pendulum were 

 comparatively larger during the first and second than during the 

 third and fourth disturbance. The change during No. III. is 

 about 1 mm. Another fact worth noting is that in the two first 

 cases the pendulum is deflected towards the south, and in the 

 two last towards the north. This seems to indicate, if one 

 considers the special arrangement of the instrument, that the 

 motion arrive I from the north in the first and second, and from 

 the south in the third and fourth case. The displacement of the 

 pendulum at Nicolaiew at 577 is also directed towards the south, 

 in accordance with the observation at Strassburg. ' The above 

 conclusion is founded on the supposition that the displacement 

 is produced by a single shock, which causes the steel-points 

 connected with the stand of the instrument to slip on the agate 

 cups. In reality, the motion is probably much more compli- 

 cated, and perhaps one is not justified in supp'>sing the direc- 

 tion of the shock to be opposite to the deflection of the pendulum. 

 The comparison of the observed times, indeed, leads to a 

 different result. 



The following table gives a summary of the observations : — 



.\gain the times of disappearance of the curve or of maximum 

 motion are separated by nearly the same interval, viz., at 

 Strassburg III. -I. = 6 S^h., IV. -II. = 7l2h.,i .and at Nico- 

 laiew IV.-II. = 69h. The duration at Strassburg of No. I. 

 is oSjh., and of No. II. o'94h. ; the duration of No. III., if 

 we omit the last part, in which the motion was very small, is 

 2S4h., of No. IV. 2'69h. .A.t Nicolaiew, during the first half 

 of the disturbance, the strong motion ends 4oSh. after the 

 beginning, and the second part Lasts 4'l2h. The intensity of 

 I. and II. is evidently larger than that of III. and IV. 



We will now see if the direction of motion can be determined 

 by the observations. 



I. Though the first trace of motion is o'loh. earlier at Nico- 

 laiew than at Strassburg, yet it is probable that the corresponding 

 moments are those of the disappearance of the curve at Strass- 

 burg and of maximum oscillation at Nicolaiew, or 4'77h. and 

 4'8h. To judge from the copy, which I'rof. Kortazii sent me, 

 the latter value is only approximate. The difference in time 

 is certainly small, and the direction of the motion remains 

 rather uncertain ; the general aspect of the figure, however, 

 makes it more probable that it came from the east. 



II. The time of disappearance of the curve at Strassburg, 

 56lh., is probably correct within oo2h. or oo3h. Mr. 

 Daviison's oliservation shows that the motion, which in this case 

 appears to have commenced suddenly, h.id not reached Bir- 

 mingham -at 5 72h.; on'the other hand, the disappearance of the 

 curve at Nicolaiew took place at 5'Sh., or about 12m. later than 

 at Strassburg. If these times were all correct, and if the three 

 moments really corresponded with the same phase, the centre of 

 disturbance ought to be looked for somewhere at the south-west 

 and not too far away from Strassburg. This, however, is a very 

 improbable result. Mr. Davison's instrument could only indicate 

 an east-west tilt, and perhaps the motion h.ad already set in when 

 he left the instrument .it 572h., but was not perceptible 

 enough in the east- west direction.^ 



The two other observations make it nearly certain that the 

 motion arrived from the west. 



III. The case is very much like No. I. : probably the motion 



Disturbance 



Strassburg. 



Nicolaiew 



I 



Birmingham 



No. I. 

 (displacement - 3"5mm) 



No. II. 



( displacement prn!iably 



i. - 48mni. 



No. III. 

 (displacement -h imm.) 



No. IV. 

 (displacement -H 10.3mm. 



4"42 first trace 



4°52 increases 4mm. 



4'69 decreases and increases again 



4'77 curve disappears 



505 reappears 



h. 

 4-32 



first trace 



5-25 end 



5 61 new shock 



curve steady 



during the 



interval 



8 '45 \ light point corrected 

 ^9-65 J motion small I ^^^^ly^j^^jy 



' 1 1 45 first increase 5mm. 



I I i°6o second increa.^e 10 mm. 



l i2'io end ) curve 



I2'26 first small motion ) steady 



1 2 47 increase 6mm. 



1273 curve disappears 



1303 reappears { ^^,jg„ j^,^,,, 



1 4 '95 new small increase 



4'S first maximum>6omm. 



5'S curve disappears 

 strong motion 



small motion 

 sudden increase 



diminishes 



8-4 



972 ( 

 1 1 OS ) 

 11-36 



12-3 



127 

 1322 



13-97 

 14-87 

 15-17 



curve disappears 

 ,, reappears 

 decrease of motion 

 new increase 

 end 



5-72 the image was found 



to be steady 

 6-48 strong motion 

 6-70-6-77 steady again 

 8-22 end 



When looking ov-'"--— •■— lut-, .i.v .> inclined to think that 

 the remarkable con between the several phases of 



the disturbance cat: ■ to chance. If we take as the 



beginning of a ditlurbance the moment when its first traces are 

 visible, we have the fallowing differences at Strassburg : — 

 II.-I. = i-igh. and 1 V.-III. - i loli , III.-I. = 6-74h., 

 IV.-II. = 6-65h. At Nicolaiew, where 1. and II., III., and IV. 

 appear as a single disturbance each, we have III. -I. = 6-73h. 



• In the hjjure the i.ulve ti displaced in an op('0»ile direction, but ifli* i» 

 Um caM l>ec.iu«e the drum it-ind» well nf the pcri'liiluin :it Nicolaiew, 

 aad ea«t of it at StraAtburg. 



arrived at Nicolaiew first, but its direction cannot be determined 

 with certainty. 



' The licRinninK of I!., tliougb sudden and sh.irp. need not neMssarily 

 coinciilc «itli the inovcmL-ni of greatest motion; in this case the dilTerence 

 IV.-II. wcmld have a smaller value. _ ■ ,. o 1 -t 



- The distance between Strassburg and fiirmlngham is about 800 kilo- 

 metre*. 1 .- . 



J [Much weight cannot be attached to the absence of observed motion al 

 BirniinKhani at 073h. The image of the wire was .nljuiied on the cross- 

 wile of the teliscopc without diffiiiilty. and must have remnincd practically 

 in contact for a lew seconds. A small movement, with a period so long as 

 twenty seconds, might easily at this time have escaped notice.— C. D. I 



NO. 1313, VOL. 51] 



