404 
BAUER. 
isoclinics. As the observations so found will rarely apply 
in sufficient number to the same epoch, use may be made of 
the London secular variation formula, as above given, for re¬ 
duction to a common epoch. I hope this matter will incite 
British terrestrial magnetists to pursue the subject fur¬ 
ther. It would be a valuable addition to our knowledge of 
the secular variation if all the observations ever made in 
England were carefully collected and discussed. 
Wliiston comes in, however, for a still greater share in the 
early terrestrial magnetic discoveries, and this has likewise 
been overlooked. It is usually asserted and believed that 
the earliest observations of relative intensity of terrestrial 
magnetic force are those of Mallet (1769), or the more sue- 
sessful ones of Lamanon (1785-’87). Mallet’s observations 
were made at two stations only, St. Petersburg and Ponoi. 
Little importance appears to have been attached to them 
even by Mallet himself. As he found the times of vibra¬ 
tion of his needle the same at both places, it was inferred 
that the magnetic force was the same at all points on the 
earth’s surface. Lamanon’s observations, the record of which 
appears to have been lost in shipwreck, were more success¬ 
ful and yielded the important discovery that the attractive 
force (horizontal) of a magnet is less in the tropics than 
towards the poles, and that the total intensity increases 
with the latitude. In consequence Humboldt has credited 
Lamanon with the discovery of this law, which was further 
confirmed by Humboldt’s own observations, 1798-1803. Of 
observations fairly trustworthy, these by Humboldt have 
been hitherto regarded as the earliest; but this distinction 
should be awarded, as I think, to Whiston, as appears from 
the following: * 
* It may be remarked here that the earliest observations with respect 
to the annual variation of the intensity appear to be by Musschenbroek 
(Phil. Trans. Roy. Soc., No. 390, p. 370, 1725). “He wished to observe 
whether the force of the magnet was the same every day or greater or 
less in summer than in winter, but he found by several experiments that 
the force is less in summer than in winter, at least in the summer of 
1725.” 
