44 



NA TURE 



[May io, 1906 



papers of February 2 announcea mat cables between 

 Jamaica and Puerto Rico had been interrupted, and on 

 later dates it was reported that severe shoclcs had been 

 felt among the West Indian islands, that six or seven sub- 

 marine cables had been broken, and that Mont Pel6e and 

 La Soufri^re, in St. Vincent, were again active. 



In concluding this short discourse, I wish to direct 

 attention to a class of phenomena from which the working 

 seismologist cannot escape. At certain times horizontal 

 pendulums may be fitfully moving continuously for hours 

 or even days. Similar movements have often been noticed 

 with balances and with other instruments. They are fre- 

 quently referred to as microseismic disturbances. Inasmuch 

 as they vary with varying meteorological conditions, and 



Fig. 2. —This is similar to Fig. t, bat refers to the year igo3, during 

 which period the pole di>placemenl was more uniform than that 

 indicated in Fig. i. 



may be different in neighbouring rooms, I am inclined to 

 think that it would be more accurate to describe these un- 

 welcome visitors, with which not only seismologists, but 

 also astronomers and others, have to contend, as air 

 tremors. When, however, these irregular movements are 

 replaced by movements which have definite periods very 

 different from those of the recording instrument itself, and 

 are at the same time regular in amplitude, it seems possible 

 that they may be connected with actual pulsatory motion of 

 the surface of the ground. 



In addition to tremors and pulsations, the records on 

 the films from seismographs show that nearly at all times 



with barometric loading. The quantity of water in wells 

 and that flowing in drains and from springs has been 

 observed to vary with fluctuations in atmospheric pressure. 

 Where this takes place, subsurface operations are revealed 

 which may be sufficient to give rise to changes in surface 

 level. Very marked changes of level take place at certain 

 stations during wet weather. In the Isle of Wight, at 

 Shide, which is situated on the side of a valley cut through 

 an anticline of chalk, when heavy rain occurs, levels and 

 horizontal pendulums indicate a tilting towards the bed of 

 the valley. An instrument on the opposite side of the valley 

 behaves in a corresponding manner. In other words, if 

 these observed movements can be regarded as extending 

 to the bed of the valley, it may be said that with rain the 

 steepness of each of its sides is increased. During fine 

 weather the direction of movement is reversed. A more 

 regular movement is, however, found in a tilting known 

 as the diurnal wave. With the same assumption as to the 

 extent of corresponding motion we find, but only during 

 fine weather, that the direction of moveinent of the sides 

 of the same valley during the night corresponds to that 

 observed during wet weather. During the day it is the 

 same as that which takes place during fine weather. For 

 convenience we may regard the valley as opening and 

 closing. Similar observations have been made on the two 

 sides of a valley which has been cut through alluvium in 

 Tokio. 



Probably an important part in the production of these 

 diurnal movements is played by the differential loading and 

 unloading of neighbouring areas by solar influences. 

 During wet weather, in virtue of subsurface percolation 

 and lateral drainage generally, the sides and bottom of a 

 valley where water-level is raised carry a greater load than 

 the bounding ridges. Under these conditions the bottom 

 of a valley may sag and its sides close inwards. During 

 fine weather, in virtue of evaporation and drainage, a 

 movement in the opposite direction may be established. 

 The fine-weather diurnal movement corresponding to the 

 opening of a valley mav find a partial explanation in the 

 removal of load by evaporation, but more particularly by 

 plant-transpiration. These activities are more pronounced 

 during the day than at night, and they tend to reduce sub- 

 surface percolation and drainage towards the bed of a 

 valley. The comparatively small retrograde nocturnal 

 movement may be partly attributed to an increase of valley 

 load at night, at which time transpiration and evaporation 

 are replaced by surface and subsurface condensation. 

 Transpiration and evaporation being at a minimum at 

 night, it may be assumed that lateral percolation and 

 surface drainage towards the bed of a valley are increased, 

 and, possibly as a consequence of this action, the volume 

 of water in certain wells and that flowing in certain streams 

 and drains has been found to be greater at night than 

 during the day. 



.Another activity which mav result in a nocturnal increase 



Df Wave Groups A t 



F in the terminal vibrati 

 : Shide, Isle of Wight. 



Scale 1 



Earthquake of January 31 



a slow change of level is taking place. For years a pier 

 may be undergoing a tilt in one direction. Besides this 

 general movement the instruments reveal the existence of 

 waves that indicate a difference in the direction of move- 

 ment in different seasons. Superimposed upon these again 

 we find records of changes of level which may be associated 

 with variations in the difference in loads on two sides of 

 an observing station. When a horizontal pendulum swings 

 towards the area of greatest atmospheric pressure it appar- 

 ently indicates a change directly or indirectlv connected 



NO. 1906. VOL. 74] 



in the subsurface flow of water is the expansion of the air 

 in soil bv the slowly descending heat of the previous day, 

 this expansion forcing soil-water into passages of easiest 

 escape. 



The explanation offered for the phenomena under con- 

 sideration may be found wanting ; but the facts remain • 

 that round the face of the globe diurnal superficial distor- 

 tions can be observed which varv in magnitude and direc- 

 tion, and that rainfall is accompanied by measurable 

 changes in the slopes of certain valleys. 



