Januabt 22, 1886.] 



SCIENCE. 



75 



rate of a fork caused by changes in temperature, 

 in the amplitude of vibration, and by the pressure 

 of the style against the paper on which the vibra- 

 tions are recorded. 



LETTERS TO THE EDITOR. 



**♦ Correspondents are requested to be as brief as possible. The 

 writer's name is in all cases t equired as proof of good faith. 



An early prediction of the decay of the obelisk. 



I give below a translation of a portion of a letter 

 from Dr. Alfred Stelzner of St. Petersburg. 



" At first I wanted to add to my remarks a com- 

 parison between the New York Needle and the Alex- 

 ander column in St. Petersburg ; for the rock of both 

 is very much alike : it agrees even down to the 

 occurrence of handsome little zircon crystals. This 

 comparison would have been made but that it would 

 have been a mournful and unpleasant croak in the 

 triumphant report of Mr. Gorringe, and therefore it 

 had to be abandoned ; but privately let it at least be 

 put on record. You know, perhaps, that the Alexan- 

 der column in St. Petersburg was transported from Fin- 

 land to St. Petersburg in the thirties of this century 

 at a senseless cost, and, with the assistance of thou- 

 sands of men, was erected, — a monument for eternal 

 ages, which should remind the beholder of a Eussian 

 monarch. But even in a few years the granite did 

 sad honor to its Finnish name of ' Eappakivi,' i.e., 

 the lazy-stone. The granite commenced to weather, 

 and weathered merrily on in spite of all technical 

 and scientific commissions ; and one can well say that 

 the years of the proud monument are numbered. It 

 is possible that they chose unsound stone, and that 

 they shook it about too much ; so that, in quarrying 

 and transporting it, it became filled with little clefts, 

 and thus gave free play to its disintegration. But 

 General Helmersen explains the affair differently. 

 The granite, he says, contains many large felspar 

 crystals. But the felspar is triclinic, and therefore 

 expands, under the great differences of temperature 

 between the St. Petersburg summer and winter, dif- 

 ferently in the directions of its three axes : hence 

 comes the crumbling, owing to the unequal molecular 

 movement thioughout the entire mass of the mono- 

 lith. If this explanation is correct, then from the 

 similarity of the rocks from Finland and Syene, and 

 the great differences between the summer and winter 

 temperature which exist also in New York, an unsus- 

 pected danger threatens the old Egyptian monolith, 

 which has always hitherto stood in a mild and equa- 

 ble climate. Perhaps, also, it will succumb to the 

 weakness of old age, for the London Needle of Cleo- 

 patra is said to be beginning already to crumble in its 

 new home. You may regard this statement as pessi- 

 mistic, but a knowledge of the experiences made else- 

 where will not injure the New-Yorkers. Perhaps it 

 will lead them to cover up the Needle there with bad 

 conductors of heat during the winter, and thus pre- 

 serve the venerable old stone monument. In any 

 case, you will agree with me that this comparison 

 should be taken into consideration ; but it will not do 

 to insert it into Mr. Gorringe's book, where it would 

 produce a discordant tone. But it is worthy of con- 

 sideration. . . . Thus I wrote in 1882, and I regret 

 that I was not mistaken. But the children of the 

 tropics, be they palms or granite columns, will not 

 stand a northern winter in the open air. For the 



rest, one will interest himself now more than formerly 

 in the observations which have been made in other 

 places. I take the liberty, therefore, of calling your 

 attention to the memoir by Struve : ' The Alexander 

 monument and the Eappakivi. A contribution to the 

 better knowledge of the Finnish granites. St. Peters- 

 burg, 1863-64."' F. E. 



Sea-level and ocean-currents. 



The recent important determination of the coast 

 and geodetic survey, by levelling up the Mississippi 

 valley and across to the Atlantic coast, that the mean 

 level of the Gulf of Mexico at the mouth of the 

 Mississippi is about one metre higher than that of 

 New York harbor; and the similar result obtained by 

 Bourdalone, by levelling across France, namely, that 

 the mean level of the harbor of Brest is 1.02 metres 

 higher than that of the Mediterranean at Marseilles, 

 — furnish an interesting subject for study, and im- 

 portant facts for explanation by physical geographers. 

 If, as it seems, the surface of the ocean is not level 

 and at rest, what are the forces which cause it to 

 deviate from a perfect level, and to have ascending 

 and descending gradients in different parts, and cur- 

 rents running in various directions ? 



There are two principal causes for this disturbance 

 of sea-level, — the one, the difference of level between 

 the equatorial and polar regions, arising from a 

 difference of temperature of the sea in the two 

 regions; and the other, the deflecting force depending 

 upon the earth's rotation. The first is the real cause 

 of disturbance, the latter being simply a modifying 

 influence of the effects of the former, which changes, 

 or tends to change, the directions of motion, but does 

 not give any addition of real force. 



According to Mr. Croll (Climate and time), as 

 deduced from the soundings of the Challenger ex- 

 pedition, if the water of the upper strata were pre- 

 vented from flowing away toward the poles, the level 

 of the ocean at the equator, on account of its greater 

 temperature, would be 4.5 feet higher than the level 

 at the parallel of greatest diversity of sea-water, 

 a little beyond the polar circle. This greater upward 

 expansion in the equatorial region, however, does not 

 change the pressure at the bottom of the sea; and 

 its initial effect is to give rise in the upper strata 

 to gradients of pressure decreasing from the equator 

 toward the poles. This causes a flow of the water of 

 these strata from the equatorial to the polar regions, 

 and this decreases a little the pressure at sea-bottom 

 in the former, and increases it in the latter, and 

 consequently gives rise to a gradient of decreasing 

 pressure, and an under-current, from the polar 

 regions toward the equator. Hence there is now an 

 interchanging circulation, a motion of the water of 

 the upper strata from the equatorial region toward 

 the poles, a very gradual settling-down of the water 

 in the higher latitudes, a return toward the equator in 

 the lower strata, and a very gradual rising-up again 

 in the lower latitudes. 



If the earth had no motion of rotation on its axis, 

 this would be simply a vertical circulation without 

 any motion either east or west. But, in consequence 

 of the deflecting force of the earth's rotation, the 

 water of the upper strata, in flowing from the lower 

 latitudes toward the poles, is deflected eastward; and 

 it retains this eastward motion until it has settled 

 down in the higher latitudes into the lower strata, 

 and has returned, perhaps, to the parallel of 35 J or 30 J , 



