Scientific Lectures. 167 



being subject to the directive influence of the earth's magnetism, was 

 it that the discovery was really made. This work is due to John 

 Dalton, the illustrious founder of the numerical laws of chemistry, 

 and his beautiful revelation affords another argument in the proof of 

 the earth itself being a great magnet. Seventy-seven years before 

 him, I find that Dr. Edmund Halley, of England, had imagined very 

 clearly the same relation, but with Halley it only remained in the 

 region of the imagination, for he never really found its embodiment 

 in the material world. Dr. Dalton's discovery is found in his 

 " Meteorological Observations and Essays, Manchester, 1793," from 

 which I give these few lines of his simple language : 



"A very moderate skill in optics was sufficient to convince him 

 that as the luminous beams at all places appear to tend toward one 

 point, about the zenith, they must, in reality, be straight beams, 

 parallel to each other, and nearly perpendicular to the horizon ; and 

 from the appearance of their breadth they must be cylindrical." 



u The length of the beams bore a very great 

 proportion to their distance from the earth, even so as to equal, or 

 perhaps surpass, the said distance." 



" Thus stood the author's knowledge and ideas upon the subject in 

 the autumn of 1792. The very grand aurora in the evening of the 

 13th of October was that which first suggested and led to the dis- 

 covery of the relation betwixt the phenomenon and the earth's 

 magnetism. When the theodolite was adjusted without doors, and 

 the needle at rest, it was next to impossible not to notice the exacti- 

 tude with which the needle pointed to the middle of the northern 

 concentric arches. Soon after, the grand dome being formed, it was 

 divided so evidently into similar parts, by the plane of the magnetic 

 meridian, that the circumstances seemed extremely improbable to be 

 fortuitous ; and a line drawn to the vortex of the dome, being in direc- 

 tion of the dipping needle, it followed, from what had been done 

 before, that the luminous beams at that time were all parallel to the 

 dipping needle. It was easily and readily recollected at the same 

 time, that former appearances had been similar to the present in tlr>s 

 respect, that the beams to the east and west had always appeared to 

 decline considerably from the perpendicular toward the south, whilst 

 those to the north and south pointed directly upward ; the inference 

 was therefore unavoidable, that the beams were guided, not by 

 gravity, but by the earth's magnetism, and the disturbance of the 

 needle that had been heretofore observed during the time of an 

 aurora, seems to put the conclusion past doubt. It was proper, how- 



