August 1 6, 1S83] 



NA TURE 



369 



broadened out to a swell so flat that it could not be 

 detected on the smoothest water excepting along shore 

 lines, where the water rose and fell like the tide. Instead 

 of a wall of water 60 feet in height we have long flat un- 

 dulations perhaps 8 feet in height, but with a distance 

 from crest to crest of more than 120 miles. 



If we turn to the effects of large earthquakes as exhi- 

 bited on the land, I think that we shall find records of 

 phenomena which are only to be explained on the assump- 

 tion of an action having taken place analogous to that 

 which takes place so often in the ocean, or an action 

 similar to that exhibited by small earthquakes and arti- 

 ficially produced disturbances if greatly exaggerated. 



As a remarkable instance of such phenomena we may 

 take the great earthquake of Lisbon on November 1, 1755. 

 In Spain, Northern Italy, the South of France and 

 Germany, Northern Africa, Madeira and other Atlantic 

 Islands, the effects of the disturbance which created so 

 much devastation in Portugal were also more or less 

 severely felt as violent movements of the soil. 



In other countries further distant, as, for instance, Great 

 Britain, Holland, Norway and Sweden, and North Ame- 

 rica, although the records are numerous, the only pheno- 

 mena which were particularly observed were the slow 

 oscillations of the waters in lakes, ponds, canals, &c. In 

 some instances the observers especially remarked that 

 there was no motion in the soil. 



Pebbly Dam in Derbyshire, which is a large body of 

 water covering some 30 acres, commenced to oscillate as 

 a strong current from the south. 



A canal near Godalming flowed 8 feet over the walk on 

 the north side. 



Coniston Water in Cumberland, which is about five 

 miles long, oscillated for about five minutes, rising a yard 

 up its shores. Near Durham a pond 40 yards long and 

 10 yards broad rose and fell about 1 foot for six or seven 

 minutes. There were four or five ebbs and flows per 

 minute. 



Loch Lomond rose and fell through about 2^ feet every 

 five minutes, and all other lochs in Scotland seem to have 

 been similarly agitated. 



At Shirbrun Castle in Oxfordshire, where the water in 

 some moats and ponds was very carefully observed, it 

 was noticed that the floods began gently, the velocity 

 then increased, till at last with great impetuosity they 

 reached their full height. Here the water remained for a 

 little while, until the ebb commenced, at first gently but 

 finally with great rapidity. At two extremities of a moat 

 about 100 yards long it was found that the sinkings and 

 risings were almost simultaneous. The motions in a 

 pond a short distance from the moat were also observed, 

 and it was found that the risings and sinkings of the two 

 did not agree. 



During these motions there were several maxima. 



These few examples of the motions of waters without 

 any record of the motions of the ground at the time of 

 the Lisbon earthquake must be taken as examples of a 

 very large number of similar observations of which we 

 have detailed accounts. 



Like agitations it must also be remembered were per- 

 ceived in North America and in Scandinavia, and if the 

 lakes of other distant countries had been provided with 

 sufficiently delicate apparatus, it is not unlikely that like 

 disturbances would have been recorded. 



The only explanation for these phenomena appears to 

 be that the short quick vibrations which had ruined so 

 many cities in Portugal had by the time that they had 

 radiated to distant countries gradually become changed 

 into long flat waves having a period of perhaps several 

 minutes, and in countries like England these pulse-like 

 movements were too gentle to be perceived excepting in the 

 effects produced by tipping up the beds of lakes and ponds. 



The phenomenon was not unlike that of a swell pro- 

 duced by a distant storm. 



At Amsterdam and other towns chandeliers in churches 

 were observed to swing. At Haarlem floods rose over the 

 sides of tubs, and it is expressly mentioned that no motion 

 was perceived in the ground. 



At the Hague a tallow chandler was surprised at the 

 clashing noise made by his candles, and this the more so 

 because no motion was felt under foot. 



At Toplitz the pulsation of the ground appears to have 

 manifested itself in effects upon the springs. The flow of 

 the principal spring was greatly increased. Before this 

 increase it became turbid, and at one time stopped. Sub- 

 sequently it became clear, and flowed as usual, but the 

 water was hotter and more strongly mineralised. 



At one or two places, as, for instance, in Britain, slight 

 earthquakes were experienced. These, however, were 

 local, and in every probability were secondary disturb- 

 ances produced by the pulsations causing ground in a 

 critical state to give way. 



In this earthquake I think, then, that we have a clear 

 case of the production of pulsations in the soil that were 

 too slow to be felt by ordinary observers. 



Motions like these might be called slow earthquakes, 

 and it does not seem unlikely that they are the resultants 

 of all large disturbances. When they accompany a large 

 earthquake like that of Lisbon, their cause is evident. 

 But when we see the waters of lakes and ponds oscil- 

 lating, the bulbs of levels disturbed, and the plummet line 

 of pendulums displaced, the reason of these phenomena are 

 not so apparent. It would seem possible that in some 

 cases pulsations producing these phenomena might have 

 their origin beneath the oceans, or deep down beneath 

 the earth's crust. Perhaps, instead of commencing with 

 the snap and jar of an earthquake, they may commence 

 as a heaving or sinking of a considerable area, which may 

 be regarded as an uncompleted effort in the establishment 

 of an earthquake or a volcano. The very fact that we 

 know that volcanoes rising from deep oceans have in the 

 first instance forced their way against a pressure of at least 

 three or four tons to the square inch, indicates to us the 

 existence of internal pressures tending to raise the crust 

 of the earth, which pressures are infinitely greater than 

 any of the pressures which we have upon the surface of 

 our earth produced by tides and variations in the baro- 

 metrical column. If we follow the views of Mr. Mallet in 

 considering that the pressures exerted on the crust of our 

 earth may in volcanic regions be roughly estimated by 

 the height of a column of lava in the volcanoes of such 

 districts, we see that in the neighbourhood of a volcano 

 like Cotopaxi the upward pressures must have been many 

 times greater than the pressures already mentioned — sea 

 level being taken as the line of hydrostatic equilibrium. 

 The chief point, however, is that beneath the crust of 

 our earth enormous pressures exist tending to cause 

 eruption ; and farther, that these are variable. Before a 

 volcano bursts forth we should expect that there would be 

 in its vicinity an upward bulging of the crust, and after 

 its formation a fall. Farther, it is not difficult to con- 

 jecture other possible means by which such pressures 

 may obtain relief. 



Should these pressures then find relief without ruptur- 

 ing the surface, it is not difficult to imagine them as the 

 originators of vast pulsations which may be recorded on 

 the surface of the earth as wave like motions of slow 

 period similar to the motions in the outer area of a tract 

 disturbed by a destructive earthquake. 



That slow, undulatory motions or changes in the ver- 

 tical do occur in the crust of the earth, whatever may be 

 their origin, we have numerous phenomena which cer- 

 tainly admit of explanation on such an assumption. 



In Switzerland from time to time we hear of oscilla- 

 tions in the waters of lakes known under the name of 

 Rhussen and Seiches. These, it may be remarked, are 

 common to the lakes and inland seas of many countries. 

 Other examples of what may have been a slow oscil- 



