lxxxii Journal of the Asiatic Society of Bengal [N.S., XII, 



Earth : this is the well-known Contraction theory illustrated in 

 this diagram ; the Earth's interior is held to cool and to con- 

 tract, and the outer crust is supposed to get too large for the 

 shrinking core and to wrinkle. 



About 1860 the observations of the plumb-line in these 

 Provinces brought to light a most important and totally un- 

 expected fact, namely that the Himalaya were not exercising 

 an attraction at all commensurate with their bulk. 



This instrument is a plumb-line. It is a simple weight 

 suspended on a string, and it hangs under the influence of the 

 attraction of the Earth which pulls it downwards : you know 

 from mechanics, that if one force pulls this weight vertically 

 and if another force pulls it horizontally, the weight will hang 

 in a resultant direction inclined to the vertical. Sixty years 

 ago the question had to be considered, how will a weight hang 

 near the foot of the Himalaya : here there will be two forces ; 

 the Earth's mass will be pulling the weight vertically, and the 

 mass of the Himalaya will pull it horizontally. You may 

 think that the mass of the Himalaya is very small compared 

 with that of the Earth ; that is true, but we can measure by 

 the stars very small angles of latitude and longitude, and the 

 question was, Will the Himalaya deflect the plumb-line suffi- 

 ciently to affect the observations of the Survey ? 



The plumb-line was observed at Kaliana, a village near 

 Muzaffarnagar in the United Provinces, 60 miles from the foot 

 of the mountains : the observers found that the Himalaya were 

 exercising no appreciable attraction. Archdeacon Pratt, the 

 mathematician, then calculated from the known dimensions of 

 the Himalaya mass the attraction that the Himalaya should 

 exercise. Geographical exploration has taught us more about 

 the dimensions of the Himalaya and Tibet than Pratt knew, 

 and Major Crosthwait has now revised his actual figures. By 

 the theory of gravitation the plumb-line ought to be deflected 

 at Kaliana 58 seconds towards the hills ; it is not deflected at 

 all. It hangs vertically. This discovery was the first contri- 

 bution made by geodesy to the study of mountains. The dis- 

 covery was this, that the Himalaya behaved as if they had no 

 mass, as if they were an empty eggshell ; they seemed to be 

 made of rock, and yet they exercised no more attraction than 

 air. From the Kaliana observations Pratt deduced his famous 

 theory of mountain compensation : he explained the Kaliana 

 mystery by assuming that the rocks underlying the mountains 

 must be lighter and less dense than those underlying plains 

 and oceans. The visible mountain masses, he said, are compen- 

 sated by deficiencies of rock underneath them. This is the 

 theory of Mountain Compensation. 



The compensation of the Himalaya is not believed now to be 

 exactly complete and perfect: they seem to be compensated to 

 the extent of about 80 per cent ; their total resultant mass is 



