TRANSACTIONS OF THE SECTIONS. 37 



as changes in the magnitude of the horizontal directive force, the "bifilar 

 apparatus " is used : a magnetic bar is strained into a position at riglit angles to 

 the magnetic meridian, by suspension by means of two wires, which (their direc- 

 tions not being parallel) exert a torsion-force in opposition to the terrestrial mag- 

 netic force : when the ten-estrial force increases or diminishes, it overcomes in _a 

 greater degree or jdelds to the torsion-force ; and thus the changes of the magni- 

 tude of terrestrial directive force in the horizontal plane exhibit themselves by 

 oscillations of the bifilar magnet, which are photographically registered in the same 

 manner as those of the free magnet. 



Having for every hour (or for any other intervals of time) the measiu-es of the 

 perturbing force in the E. and W. direction, and of that in the N. and S. direction, 

 we can compound these two forces by the mechanical law of " composition of forces," 

 and can assign the magnitude and the direction of the entire perturbing force (in the 

 horizontal plane) which acts upon the magnet. 



Upon discussing these pertmujiug forces, the following conclusions were obtained : 



1. The mean annual diminution of western magnetic declination is about 7 '-9; 

 and the mean annual increase of horizontal directive force is about ^^th part of 

 the whole horizontal force. 



2. The diurnal inequalities diminish gradually through the period 1848-1857 ; 

 the proportion of their magnitude at the end of the period to that at the beginning 

 being about 3 : 5. This seems to show a general diminution of the power of the 

 Sim. 



3. The diurnal inequalities are greater in summer than in winter, in the propor- 

 tion of 5 : 3 nearly. 



4. When the means for the 24 hours of the day are taken, the westerly declina- 

 tion is increased in summer, and the horizontal force is diminished, in a greater 

 degree than corresponds to uniform change according to the law of conclusion 

 No. 1. 



5. "VMien we form a curve by moans of polar coordinates, drawing, from a zero 

 point, lines in the direction of the perturbing force acting upon the north end of the 

 magnet at every hour of the day, and with length proportional to the magnitude of 

 that force at every hour (as derived from the mean of all the observations at each 

 hour), an elliptical curve is produced, greatly extended in the direction of S.W. 

 (which point corresponds to the distiu-bing force at 1 p.m.), and much less extended 

 in other directions. 



6. The great distui'bance of the magnet occurs therefore when the sim is nearly 

 vertical on the North Atlantic Ocean, and is directed towards the point to which 

 he is nearly vertical. 



7. Combining this conclusion with conclusions Nos. 3 and 4, the Astronomer 

 Royal expressed himself as fully persuaded that the diurnal changes at Greenwich 

 are produced by the attraction of the North Atlantic Ocean, when the sun radiates 

 strongly upon it, for the north end of the needle ; the attraction of the continent 

 of Africa, when the sun shines upon it, being in comparison very small. 



8. The curve mentioned in No. 5 has some distortions of singular character cor- 

 responding to the hours of night, which are not fidly explained. 



9. The Astronomer Royal then explained that the observations had also been 

 discussed for discovery of the pertu^rbatious following the law of the lunar positions 

 with respect to the meridian. The general result appeared to be that, twice in 

 every lunar day, there is a force directed towards Hudson's Bay. Tiiere are some 

 anomalies in the partial results ; and the Astronomer Royal expressed himself as 

 not very confident on the accuracy of the law, and not very distinct in his views of 

 the explanation. The form of the general result bears a strong analogy with that 

 of Tides of the Sea. 



On the Formation of Standards of Electrical Quantity and Resistance^ 

 By Latimer Clark and Sir Charles Bright. 



The object of this paper was to point out the desirability of the estabrshment of 

 a set of standards of electrical measurement, and to ask the aid and authority of the 

 British Association in introducing such standards into pi'actical use. Four stand- 

 ards or units were considered necessary. 



