148 Local Attraction. [No. 2, 



Four years after this, following up Mr. Airy's suggestion, I pro- 

 posed and reduced to calculation another hypothesis regarding defi- 

 ciency of matter below the mountains ; viz. that the irregularities 

 of the mountain surface have arisen from the expansion upwards of 

 the crust of the earth from depths below, which has upheaved the 

 mountains and produced a slight but extensive attenuation of the 

 mass below them. The result of this calculation is given in the 

 Second Paper of this series. I show that it is sufficient to produce 

 a considerable amount of compensation for mountain attraction ; but 

 that it does not clear up the difficulties ; and that as this attenuation 

 is a mere hypothesis, nothing certain can be determined regarding it. 



In this same paper it is shown that a very slight but wide-spread 

 defect or excess of density in the materials of the crust of the earth 

 is capable of producing a sensible and important effect on the plumb- 

 line. Thus the possible and not improbable existence of an unknown 

 cause of derangement of the plumb-line hitherto unthought of, as 

 being hidden in the crust, was brought to light. 



During the same year it occurred to me that there is another 

 visible cause of disturbance besides the mountains which might pro- 

 duce a sensible effect, viz. the ocean, as its density is less than that 

 of rock. In the Third Paper this effect is calculated, and found 

 to be of importance : (see Phil. Trans, for 1858, p. 790, art. 1J). 

 Thus a new source of error was detected. 



3. Thus far, then, the attempt to clear up the discrepancies de- 

 tected in the first instance by Colonel Everest between the astro- 

 nomical and geodetical amplitudes had led to the discovery, that (1) 

 the Himalayas attract places in the plains of India with a force far 

 greater in amount than any person had conceived : And not only so, 

 but that (2) the ocean also has an important influence of the same 

 kind : And more than this, that (3) variations of density in the 

 crust, which are as likely to exist as not, will produce the same effect. 



The uncertainty, as to the form of the Himalayas and the depth 

 of the ocean, produces a corresponding degree of uncertainty as to 

 the exact amount of the attraction ; while our utter ignorance re- 

 garding the condition of the crust below seemed to leave us in hope- 

 less perplexity regarding the derangement which may proceed from 

 that quarter. So that the attempt to determine the resultant amount 

 of local attraction at stations on the Indian Arc by direct calculation 

 would appear, for these reasons, altogether fruitless. 



