50 OLDHAM: THE STRUCTURE OF THE HIMALAYAS, ETC. 



and the defect in support taken up by a depression of the crust on 

 either side by the equivalent of 1,500 ft. gradually diminishing to 

 nothing in 200 miles. In each case the figures given in table No. 10 

 must be added, algebraically, to those given in table No. 9 for the 

 hypothesis of support by simple flotation, and so will increase or 

 diminish the differences from the deflections due to the Hayford 

 compensation, as the case may be. 



The effect of the opposite supposition, that the buoyancy of the 

 downward protuberance is in excess, and the surplus power of flota- 

 tion absorbed by an upward bending of the crust on either side, would 

 be practically the same in amount, but with the opposite sign, as that 

 shown in the table No. 10, the surplus buoyancy being supposed to be 

 of equal amount and extent as the surplus load considered in that table. 



In considering the gravity observations a somewhat different 

 course to that adopted in the case of the deflection of the plumb- 

 line will be more convenient. The effect of the direct attraction 

 of the visible masses is always determinable from the published 

 observations, and different formulae of calculation make very small 

 differences in the amount to be allowed for this effect ; the anomalies, 

 or more properly the difference of anomaly between two stations 

 in the same region, may therefore be looked upon as representing 

 local differences in the density of the matter under the station, 

 of which the most important is that due to the effect of compensa- 

 tion. It is, consequently, convenient to consider the effect of the 

 compensation only, and the differences which would be introduced 

 by varying the hypothesis. 



The first of these comparisons to be made is that of the Hayford 

 compensation with an hypothesis of support by flotation. This is 

 given in table No. 11 (page 51), and a few words of explanation 

 will show the use of this and the other tables; taking station 0, at 

 the edge of the hills, and calculating the gravity which should be 

 found at it according to the Hayford factors, we would have to allow 

 for the effect of the visible masses and a further correction of — '075 

 dyne for the effect of their compensation ; but if the support had in 

 reality been, as considered in the second column, one of simple flotation 

 its effect would have amounted to — '085 dyne, and the observed 

 force of gravity would show a defect, or anomaly, of — '010 dyne. 

 At stations more than 50 miles into the hills this would be reversed, 

 and a calculation based on the Hayford tables would show a 



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