484 P)'of. John Milne — On Earth Movements. 



the tides in the ocean. From time to time we read of the collapse 

 of underground excavations in mines, resulting in the loss of life and 

 property. We may reasonably ask whether these catastrophes do 

 not hold some relation to the palpitations of our earth's crust, which 

 hitherto have not been investigated. In instances like these, may 

 they not have been analogous to the last straw on the camel's back 

 and the ultimate cause of ruin and destruction ? 



To point out the rewards, not only scientific, but also directly 

 utilitarian, which might accrue from the systematic investigation of 

 earth tremors, is at present only a task involving speculations. 



These movements of the ground on which we dwell being 

 phenomena so universal, and which promise such rich rewards to 

 those who successfully prosecute their investigation, it surely becomes 

 our duty to establish the necessary means for their examination. 



After the earth tremors we have to consider the existence of a 

 class of movements on our soil which yet require further observation 

 . before we are assured that they can be recorded experimentally. 

 These are similar to those spoken of by Mr. George Darwin, who 

 has shown us that an increase in barometrical pressure over an area 

 is equivalent to loading that area with a weight, in consequence of 

 which it would be depressed. Sir William Thomson likens this 

 phenomenon to placing a pile of sovereigns on a mass of jelly. 

 When these loads are removed the depressed area rises, and an 

 oscillation of the earth's crust has been completed. Earth tremors 

 have been unnoticed by ordinary observers because the amplitude of 

 their motion was so small ; oscillations like these have been over- 

 looked because the period of their vibration is so long. Whether 

 movements analogous to these, which may be caused by atmospheric 

 pressure, and which are of great amplitude and long duration, exist 

 in nature, is at present to some extent problematical. That they 

 exist and have a connexion with many phenomena which are at 

 present unintelligible is not improbable. Among these phenomena 

 may be mentioned the abrupt oscillation in the levels of water 

 which, from time to time, have taken place in inland lakes. Thus 

 we have the Seiches and Buhssen of Switzerland in lakes like Geneva 

 and Constance, where the waters, for reasons without definite ex- 

 planation, rise suddenly through a distance ranging from a few 

 inches to a yard. The like phenomena exist in the Baltic, in the 

 great lakes of America, and, probably, all over our globe. Similar 

 phenomena appear sometimes to have been connected with great 

 earthquakes, in countries at long distances from the scene of 

 destruction, and the water in lakes and ponds has slowly run back- 

 wards and forwards without any motion being perceptible in the 

 ground by those who observed the strange movement of the water. 

 This appears to have been the case in England in 1755, on the day 

 of the great Lisbon earthquake, when the water in the lakes and 

 pools oscillated to and fro like the water in a basin which is being 

 slowly tilted. At Lisbon the vibrations of the ground were rapid 

 and severe ; but when they reached England it seems probable that 

 the period of motion had been so far lengthened that the vibration 



