250 Mr. Airy — Comparison of Magnetic Disturbances ^c. [Feb. 6, 



had been anticipated. From 1865, April 18, it has been continuously in 

 use, in the same form as at present. 



The author then gives the theory, algebraical and numerical, for infer- 

 ring, from the magnitude of the galvanic currents observed in two known 

 directions, the magnitude of the galvanic currents in the north and west 

 directions. And, proceeding from these by the known law, that when a 

 current from the graphite or copper pole of a battery passes under a 

 needle, it forces the austral element to the right (as referred to the current- 

 course), he infers the magnitude of the magnetic force in the north and 

 west directions. 



The numerical expression contains four unequal constant factors, by 

 which the photographic ordinates must be multiplied. The author 

 explains that, for his own preliminary examination, he used four pro- 

 portional compasses, constructed expressly for this purpose by Mr. Simms, 

 and thus formed the ordinates of the new Magnetic-Force Curves with- 

 out any use of numbers whatever. But for the more detailed work to 

 be done by young assistants, he judged it better to measure the ordinates 

 by scales with graduations of different vahie, and to add the results, thus 

 forming numerical values of the magnetic ordinates. 



The resulting scale being arbitrary, it was so adapted that the largest 

 ordinates were not very different from the largest ordinates of the curves 

 given by the Horizontal-Force Magnetometer. The curves given by the 

 Declination Magnetometer were adapted to the scale of the Horizontal- 

 Force Magnetometer. 



In the large diagrams exhibited to the Society, the curves representing 

 the North Force as shown by the Horizontal-Force Magnetometer, and the 

 North Force as inferred from the Galvanometers, are brought into juxta- 

 position, and the curves representing the West Force as shown by the De- 

 clination Magnetometer, and as inferred from the Galvanometers, are 

 brought into juxtaposition, for seventeen days in 1865, 1866, and 1867. 

 And the general agreement between the curves of the two classes, espe- 

 cially for the North Force, is so remarkable that the author expresses his 

 undoubting belief that the irregularities of magnetic force are caused by 

 the galvanic currents. At the same time he indicates some discordances 

 which require further examination. One of these is, that the disturbance 

 inferred from the galvanic currents usually (but not always) precedes that 

 recorded by the magnetometers. Another is, that the North Force ap-; 

 pears, from the galvanic currents, to be increased (whereas, in magnetic 

 storms, it is usually found to be diminished). There are other points of 

 smaller importance. 



The author suggests as possible that these discordances may arise from 

 the circumstance that the Observatory is at the end of each of the wires ; 

 and therefore the galvanic current which is recorded, being that which 

 covers a space whose centre is several miles from the Observatory, may not 

 correspond to the magnetic forces which are observed at the Observatory, 



