434 



jVA TURE 



[Septkmber 5, 1S95 



which the pendulum stood, and the effect of elevation 

 above the sea, were then allowed for, and the vibration 

 number, when so corrected, was regarded as the vibration 

 number for that station when reduced to the sea level. 

 The pendulum used would have made S6,ooo vibrations 

 in twenty-four hours at the equator. It must therefore 

 have been slightly longer than a seconds pendulum, 

 which would make 86.400 in the same interval. The 

 observations showed that there was a more or less marked 

 deficiency of gravity over the whole continent of India, 

 and that the deficiency was greatest at the most lofty 

 stations. -At Mor^, 15,408 feet above the sea, the deficiency 

 was enough to make the vibrations in twenty-four hours 

 twenty-four fewer than they ought to have been if the 

 attraction of the mountain had produced its full effect. 

 It was obvious, therefore, that some hidden cause existed 

 which counteracted the attraction of the mountain, 

 and this could have been no other than a deficiency of 

 density in the matter beneath it. The conclusion is 

 identical with that reached by .\iry in connection with 

 the deflection of the plumb-line, namely, that the Hima- 

 layan range is supported by a downward protuberance, 

 projecting into a more dense substratum. 



This mode of suppon, as already remarked, is similar 

 to what is termed hydrostatic equilibrium. -As applied to 

 the support of the earth's crust .American geologists have 

 given to it the name " isostacy," w hich well describes the 

 phenomenon. 



During the past year an extensive series of gravity 

 measurements has been carried out by the Coast and | 

 Geodetic Survey of the United States, by the use of the 

 half-second's pendulum, a much smaller and more 

 portable instrument for the determination of gravity than 

 any hitherto employed. Observations were made at 

 twenty-six stations, eighteen of which follow nearly along 

 the 39lh parallel of latitude ; and these are particularly 

 well adapted to throw light on important questions 

 regarding the condition of the earth's crusl. 



"This line of stations, commencing at the .Atlantic 

 coast, ascends to near the Appalachians, traverses the 

 great central plain, gradually increasing in altitude from 

 495 106041 feet, then rises to the high elevation of the 

 main chain of the Rocky Mountain^, reaching an altitude 

 of 14,085 feet at l^ike's Peak, descends into the eroded 

 valleys of the Grand and Green Rivers, crosses the 

 summit of the Wasatch ridge, and finally descends to the 

 great western plateau of the continent." 



This scries of gravity determinations affords an ex- 

 ceptionally favourable opportunity of helping to determine 

 whether the support of the elevated regions travcrsod 

 apjjcars to be best accounted for by rigidity in the 

 foundations on which they rest, so that, in spite of 

 their weight and the largeness of the area occupied by 

 them, they are prevented from sinking down into the 

 material beneath ; or, on the other hand, whether they 

 are supported, as we have said that .Airy suggested, 

 namely by flouting in a denser substratum, or, as the 

 Americans say, by " isostacy," which is the same thing 

 as hydrostatic equilibrium. 



The general principle of the method pursued In re- 

 ducing gravity to the sea level has been already explained. 

 It consists In adding a correction equivalent to the 

 diminution of gravity due lo the elevation of the station, 



NO. 1349, VOL. 52] 



and subtracting a correction cquiv.ilcnt to the attraction 

 of the mass of the elevated plain upon which the station 

 may be considered to be situated. When these two 

 corrections have been made, gravity so corrected would 

 be the same as that appropriate to the latitude, or, as it 

 may be tenned, to the "computed value," unless there is 

 some deviation from regularity in the density of the 

 matter below sea level. The result proved that this was 

 the case. For gravity so reduced turned out to be 

 invariably less than that appropriate to the latitude. It 

 was clear, therefore, that at these stations in America 

 there was a deficiency in density beneath the elevated 

 districts, just as had already been found to be the case 

 in India. There could be no doubt that isostacy 

 had a share in contributing to their support. The inquiry 

 was now carried a step further. Did each mountain 

 individually owe its support to a separate protuberance of 

 its own beneath it, or was the mountainous region as a 

 whole supported in that manner, each separate mountain 

 o«ing its support to the strength of the crust on which it 

 was a mere5;excrcscence ? The case might be illustrated 

 by conceiving a number of logs of wood of different sizes. 

 If these float side by side in water, the larger logs will 

 stand the higher above the surface of the water ; but each 

 log will have a part immersed which will be its individual 

 support, and this will be deeper for the logs which stand 

 the higher. Hut if these logs are placed upon a raft, the 

 support will be general, and derived from the support of 

 the part immersed of the entire raft, and its depth will 

 depend upon the aggregate weight of the logs. Never- 

 theless it need not dip deepest beneath the logs which 

 stand the highest above the water, or above the floor of 

 the raft. 



The presumption was against each elevation being 

 separately isostatically supported, because the deficiency 

 in gravity, and therefore in density, was not found to be 

 greatest precisely beneath tlie highest stations. To carry 

 out the inquiry more fully, it was considered that, by 

 omitting the part of the reduction to the sea level whicli 

 takes account of the attraction of the mass of the plain 

 (which would mean omitting to subtract the attraction 

 produced by it), we should, as it were, transfer its mass to 

 the suljjacent parts, and so make up for the lack of 

 density, and obtain the condition of uniform density below 

 the sea level. There would then remain only the correc- 

 tion for elevation necessary. If this proceeding gave the 

 value appropriate to the latitude under each station, it 

 would show that the individual stations were seriatim in 

 isostatic equllibriuin. Hut the attempt failed. It was 

 found that the attraction of the matter of the more 

 elevated stations was not separately compensated by 

 defect of density immediately below. The analogy of the 

 detached floating logs did not hold good. It remained to 

 inquire whether the series of stations was In isostatic 

 ei|tiilibrium when considered as a whole — the case more 

 nearly analogous to the raft. If this were so, gravity, 

 when reduced to the sea level, would be uniform for 

 the whole tract. 



Fortius purpose a mode of reduction devised by M. Faye 

 was adopted. The altitude of the country surrounding the 

 station within a radius of 100 miles was reduced to a mean 

 altitude, and the attraction of a plate of rock of thickness 

 equal to the difference of altitude between this mean 



