October 29, 1886.] 



SCIEJsrCE. 



391 



actual space it has occupied for ages. This rock is 

 like hard glass, elastic, which involves compressibility. 

 At last the compressive stress accumulatiog for ages 

 becomes too great to be borne without relief, which 

 can come only from fracture. 



The fi-acture, once started, extends from its initial 

 point in lines of dislocation, as is in cold countries 

 constantly observed in the thick ice covering lakes, 

 and as is seen in the heated pane of glass. 



But the commotion, the shock, the rending, the 

 noises, are infinitely greater than in the case of the 

 pane of glass or the sheet of ice. In the sudden 

 splitting, rending and jarring dislocation of the glass, 

 we have the working model of the heated strata of 

 rock. If the effect bears any proportion to the 

 relative magnitude of the model and the rock, then 

 we have force, stress, movement, noise enough to 

 produce all the audible and visible effects of the 

 Charleston earthquakes. 



The sudden dislocation and displacement under 

 Charleston may produce the local shock ; the noise of 

 the sudden splitting of the rock in place, the sound 

 like distant cannon-shot. The long roar and grind- 

 ing, like ten thousand rusty iron chariots on a rocky 

 road, may be due to the production of a crack, which, 

 if ten miles long, and instantaneous throughout its 

 whole length, would yet be heard only as the sound 

 from each foot of its length arrived at the ear of the 

 hearer. The sound produced under foot might be 

 heard within a few seconds ; and that produced fifty 

 or sixty thousand feet away, say ten miles, would 

 not reach the ear till it was fifty or sixty seconds old ; 

 and, as the sound of successive portions breaking at 

 different distances arrived, there would result a con- 

 tmuous and heavy roar. Such a dislocation would 

 relieve in great measure the general, the widely dif- 

 fused stress and strain. But movements would be 

 loc'dl as well as general, and the smaller but still im 

 mer.se sections of our stratum of rock might continue 

 for days and weeks to adjust themselves by smaller 

 cracks, crushings. and dislocations, producing the 

 lesser shocks, sounds, and roars which commonly 

 follow the first and greatest disturbance. Such have 

 followed that of Charleston and Summerville. In 

 fact, the pane of thick glass breaking over the flame 

 of an alcohol lamp in the laboratory or on the lecture- 

 table seems to give a working- model, illustrating all 

 the known and reported phenomena of the Charles- 

 ton earthquake. The heat supposed to be observed 

 by some in the ejection of water and mud may well 

 have come from the sudden compression and stresses 

 set up in the moment of dislocation. Sudden shocks, 

 compressive stresses, and motion arrested, produce 

 heat, as, when a fifteen-inch cast-iron ball at great 

 velocity breaks to pieces against an iron target, its 

 scattered fragments are all hot to the hand that 

 gathers them. Ten miles square of hard limestone, 

 if heated 10°, would expand three feet in length and 

 width if free to move ; heated 100°, it would expand 

 about thirty feet each way. Here are force and 

 movement enough to wreck a dozen Charlestons. 

 All we need on this theory is a change of tempera- 

 ture not very great nor rapid. 



Such changes are plainly registered in the famous 

 three columns of Pozzuoli described by Lyell, which, 

 having been erected above the level of the ocean, 

 have, two or three times within the historic period, 

 sunk below its surface, and been bored at various 

 levtrls by stone-boring shell-fish (Simaceae saxophagi). 

 and then risen again till these marks, undoubtedly 



made under water, are now above the water, which 

 merely bathes the floor of the temp^le, and on which 

 they still stand upright, as though never di.-turbed. 

 Lyell's clear description assigns these evident changes 

 of level to local changes of temperature in the crust 

 of the earth below Pozzuoli. Visible motion and 

 fracture of rocks also accompany the phenomena of 

 'creeping' in coal-mines. M. C. Meigs. 



Washington, D.C., Oct. 20. 



Sea-level and ocean-currents. 



I have just received a letter from my friend, Cajat. 

 John Brown, son of John Brown the martyr, which 



I have thought would interest yoiir readers in itself, 

 and furnish a better illustration than I have before 

 given of the jjower of wind-friction to move great 

 bodies of water. I therefore enclose you the follow- 

 ing copy : — 



Put-in Bay Is., Oct. 16, 1886. 



My dear friend, — At 11 o'clock Thursday even- 

 ing, the lith inst., I witnessed here a remarkable 

 fact, the effect of the late tremendous wind-storm. 

 This commenced about 7 a.m., and began to let uid at 



II o'clock in the evening, or a little later. I then 

 went down to the shore in front of my house, and 

 found the lake lower than the average by fully six 

 feet! This is the greatest depression from such 

 cause I have noticed during a residence here of 

 nearly twenty-four years. We have not, within this 

 period, had such a high wind steadily continued for 

 so long a time. 



The captain of the steamer Chief Justice Waite, 

 running between Toledo and the islands, reports the 

 fall of water-level at Toledo as about eight feet. 



Ever yours, John Brown, Jr. 



The rej)ly of Mr. Ferrel, contained in Science of 

 July 30, seems to me to obscure rather than illumi- 

 nate the subject it discusses. The question before us 

 is, not whether the wind has the power of raising the 

 water-level on a coast, but whether wind-friction can, 

 in the great equatorial belt and in the track of the 

 Gulf Stream, produce the flow of water ^^'hich is there 

 observed. The striking cases of the power of wind 

 to heap water on coasts, and to move bodily great 

 masses of it in lakes, are only interesting and relevant 

 as demonstrating the sufficiency of wind-friction to 

 produce broad and rapid surface-currents. This con- 

 ceded, and the case is won, because, in the lakes and 

 open ocean, like catises i^roduce like effects. Wind 

 of given velocity raises in both places waves of equal 

 height in equal times : against these waves the wind 

 presses in the direction of its flow, with no oj^posing 

 force. As a consequence, the roughened water-sur- 

 face, from greatly increased friction, is moved bodily 

 forward just as though impelled by the paddles of a 

 revolving wheel. This surface-flow is in time com- 

 municated to underlying strata, and, if the wind con- 

 tinue to blow in the same direction, ultimately a 

 large body of water will be set in motion ; in oth«r 

 words, an ocean-current will be produced. There is 

 no escape from this conclusion ; and all that part of 

 Mr. Ferrel's paper which relates to wind-velocities, 

 gradients, cross-sections, etc., are irrelevant. The 

 great truth remains, that wind-friction can produce 

 ocean-currents. The difference in specific gravity 

 between cold arctic and warm troiiical water is un- 

 doubtedly also a -vera causa, the only difference be- 

 tween Mr. Ferrel and myself being as to the relative 

 value of these two factors. Impressed as I am T\dth 



