Dr. Lamont on the most advantageous Form of Magnets. 369 



index from one end of the scale to the other, while the eye is 

 applied to the telescope. The spectrum will then be seen to 

 move with varying velocity. It will seem at first to move fast, 

 and then gradually to diminish in velocity till the middle of the 

 scale is reached, when it will be for the moment stationary, 

 and then begin to move slowly in the same direction as before 

 and with ever-increasing velocity till the end of the scale is 

 reached. This also accounts for a fact which every one who has 

 used the instrument must have observed, viz. the great prepon- 

 derance of green in the spectrum thrown by the sun or common 

 gaslight. It is obvious that the knowledge of these results gives 

 a method of magnifying any portion of the spectrum, and of de- 

 termining its limits with greater accuracy. 



XLVI. On the most advantageous Form of Magnets. 



By Dr. Lamont*. 



[With a Plate.] 



THE further the science of magnetism advances in its deve- 

 lopment, the more important becomes the decision of the 

 question what form should be given to the magnets in order to ob- 

 tain the most advantageous effect. If we at present confine our- 

 selves to those magnets which have been employed in the in- 

 vestigation of the magnetism of the earth, we find that by some 

 observers very acute sharp-pointed needles, by others flat pris- 

 matic needles, have been pronounced to be the best adapted to 

 the purpose ; solid or hollow cylinders also have been recom- 

 mended. I am not aware, however, that experiments of a dis- 

 criminating character have as yet been carried out ; nor, so far 

 as I know, have even the principles been established accord- 

 ing to which the preference of one form over the others is to be 

 determined. Nevertheless, as regards the latter point, a nearer 

 consideration will show that scarcely an uncertainty or difference 

 of opinion can exist, as there are in general only three subjects 

 of observation which come into consideration in magnets, and it 

 cannot be doubtful in what relations to the result these determi- 

 nations stand. 



The three determinations here referred to are — the magnetic 

 moment, the weight or the mass, and the moment of inertia ; and 

 that form is to be recognized as the most advantageous, in which 

 are united the greatest possible magnetic moment, with the 

 smallest possible mass, and the smallest possible moment of 

 inertia. 



The direct way to decide upon the most advantageous form of 



* Translated from Poggendorff's Annalen, vol. cxiii. pp. 239-249. 

 Communicated by the Astronomer Royal. 



