ON THE MAGNETIC SURVEY OF SCOTLAND. 



175 



t Obtained through the kindness of Colonel James. 



* Obtained through the kindness of Mr. Stanford, Charing Cross. 



Let us now divide the stations according to their geological character into 

 two groups, the first group comprising the unstratified rocks, trap, and 

 granite ; and the second every other formation. We shall find that there 

 are thirteen stations, including Tobermorie, in the former, and forty-one 

 stations in the latter group. 



The sum of the squares of the differences between the observed and cal- 

 culated dip for the thirteen stations is 4394' - 2, and, consequently, the mean 

 probable error is 12' - 9. 



If we exclude Tobermorie, these numbers are 1053 ,- 4, 6'6. For the 

 group of forty-one stations we have the sum of squares = 24 86'" 1, and the 

 mean probable error=5' - 3. 



We thus see that, whether we include Tobermorie or leave it out, the 

 mean probable error of dip for those stations in the neighbourhood of igneous 

 rocks is greater than for those where the formations are of a stratified 

 description. 



Division II. — Total Force. 



These observations were taken by two different methods. 



1. Method of deflections and vibrations. — The instruments here used, and 

 the method of observation, are already so well known, that it is unnecessary 

 to describe them. Full details regarding these will be found in the Ad- 

 miralty Manual of Scientific Inquiry, 3rd edit., 1859. By means of deflec- 

 tions,-- , or the ratio between the magnetic moment of the magnet used and 

 A 



the earth's horizontal force at any station, is obtained, and by means of 

 vibrations we obtain m X or the product of the same quantities. Having 



