180 



REPORT 1859. 



Table XI. {continued.) 



If we divide the stations as before into two groups, the first comprising 

 trap and granite, and the second every other formation, we shall find that 

 there are twelve stations, including Glenmorven, in the former, and thirty- 

 one stations, including Loch Inver, in the latter class. 



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

 culated force for the twelve stations is = *0915 (British units), and, con- 

 sequently, the mean probable error is "061. 



If we exclude Glenmorven, these numbers are '0202, -030. 



For the group of thirty-one stations we have — 



Sum of squares = '05 7 7 ; mean probable error="030. 



If we exclude Loch Inver, these numbers are "0270, "020. 



"We thus see, that whether we include Glenmorven and Loch Inver or 

 leave them out, the mean error of force for those stations in the neighbour- 

 hood of igneous rocks is greater than for those where the formations are of 

 a stratified description. 



In the second map attached to this Report (Plate 7), the isodynamic lines 

 for 1st January, 1837, are compared with those for 1st January, 1858 ; the 

 force at London being reckoned = unity in both cases. It will be noticed, 

 however, that we have no record of the absolute change that has taken 

 place in the total force between the two epochs, as we have no absolute 

 measure of the force at London for 1st January, 1837. 



2. Dr. Lloyd's Statical Method. — In this method the dip circle is 

 employed. A needle is loaded with a small weight, and its position of equi- 

 librium enables us to find the product of its magnetic moment into the 

 earth's magnetic force. The needle is then removed and attached to an arm 

 at right angles to that which carries the microscopes, it being now used to 

 deflect another needle substituted in its former place. 



The moveable arm is next turned round until the deflected needle assumes 

 a position at right angles to the deflecting needle, so that the extremities of 

 the former are viewed by the microscopes. The position of the deflected 

 needle enables us to find the ratio between the magnetic moment of the 

 deflecting needle and the earth's magnetic force. A detailed description of 

 his instrument is given by the Rev. H. Lloyd in the Transactions of the Royal 

 Irish Academy for 1858. 



