49 



We will consider what the effect of a mountain will be. I have 

 got this instrument, to which I have referred, and it is in a position 

 here to observe a star. Suppose all on a sudden you were to 

 introduce a mountain here. Now, the efifect of that upon the 

 instrument would be this. The position of this instrument must 

 be determined— that is to say, the position of its fundamental 

 bearing must be determined — by a plumb-line or a level, which is 

 pulled down by the earth's attraction. If the mountain were not 

 there, the only force upon the plumb-line would be downwards. 

 But if you had a mountain here, the force would be a combination 

 of two, one pulling down and the other towards the mountain, so 

 that upon the whole, the plumb-line by which this instrument is 

 regulated does not take the same direction as when there is no 

 mountain. In like manner, if the instnmient is carried to the 

 other side of the mountain, the plumb-line, instead of being drawn 

 down, is drawn in an inclined direction, being attracted by the 

 mountain. Therefore, we have a disturbance of the plumb-line 

 produced by the circumstance of the mountain being here, which 

 disturbance can be determined by observations of a star made on 

 the opposite sides of the mountain. The real difference in the 

 slope of the earth's surface on the opposite sides is to be compared 

 with what the difference would have been between the two situ- 

 ations supposing the mountain had never existed. The observation 

 is one of great accuracy. At Schiehallien the difference was eleven 

 or twelve seconds,— I do not recollect the exact quantity, but there 

 was no doubt at all as to its reality and its amount. Having got 

 that, there is this proposition to be considered. If the mountain 

 Schiehallien, whose dimensions we can measure, pulls a plumb- 

 hne aside so far, what is the proportion of its attracting mass to 

 the attracting mass of the earth 1 This may be a little obscure to 

 persons who have not made any observations such as are familiar 

 to surveyors. As you see, the two stations of the instruments are 

 so far apart that the position of their plumb-lines ought to have 

 differed we will say by sixty seconds. But it is found with this 

 mountain between them, that the positions of the plumb-lines 

 differ seventy-two seconds, or perhaps more. The comparison of 



