125 



apparatus, which Saussure calls a magnetome- 

 ter*, and which Mr. Paul constructed for me at 

 Geneva. I am inclined to think, that the varia- 

 tions of intensity, believed to have been seen in the 

 same place, by means of this complicated instru^ 

 ment, were the effect of involuntary illusion. Mr. 

 de Saussure thought, that the magnetic force 

 diminished both on the mountains and during the 

 great heats of summer, while Mr. Blondeanf- 

 believed, that he had found by an instrument of 

 his own invention, that a high temperature of the 

 atmosphere increased the intensity of the mag- 

 netism. Neither of these assertions has been 

 confirmed by accurate experiments. There is 

 no doubt, that periodical variations in the in- 

 tensity of the magnetic forces exist in the same 

 place ; as have already been discovered in the 

 variation, and even to a certain point in the 

 dip;*: of the magnetic needle ; but these varia- 



* Voyage dans les Alpes, § 458 and 2103. I find the first 

 notion of a magnetometrical apparatus in Hooke's posthu- 

 mous works. This natural philosopher, a man of extraordi- 

 nary sagacity, thought, in 1680, of measuring, by means of 

 a steelyard (statera), the force with which a magnet attracts 

 iron at different distances. Posth. Works, p. 23. See also 

 Brook Taylor's experiments, made in 1715, Phil, Trans, vol. 

 xxxi, p. 204. 



+ On the apparatus, which Mr. Blondeau called magneto- 

 meter, before Saussure, see Mem. de TAcademie de la Marine 

 de Brest, t. i, p. 421 . 



J Horary and diurnal variations of the dip have not yet 



2 e 2 



