62 COSMOS. 



help of instruments possessing very unequal degrees of exact- 

 ness, to establish the previously neglected, but very important 

 element of inclination at various intervals of time, and at 

 many different points ; the observations being made more at 

 sea and in the immediate vicinity of the ocean than in the 

 interior of continents. Towards the close of the 18th 

 century, the stationary observations of declination which 

 were made by Cassini, Gilpin, and Beaufoy (from 1784 to 

 1790), with more perfect instruments, showed definitely that 

 there is a periodical influence at different hours of the day, 

 no less than at different seasons of the year, a discovery 

 which imparted a new stimulus to magnetic investigations. 



In the 19th century, half of which has now expired, this 

 increased activity has assumed a special character differing 

 from any that has preceded it. We refer to the almost 

 simultaneous advance that has been made in all branches of 

 the theory of terrestrial magnetism, comprising the numerical 

 determination of the intensity, inclination and variation of 

 the force ; in physical discoveries in respect to the excitation 

 and the amount of the distribution of magnetism ; and in 

 the first and brilliant suggestions of a theory of terrestrial 

 magnetism, which has been based by its founder, Friedrich 

 Gauss, upon strictly mathematical combinations. The means 

 which have led to these results are improvements in the 

 instruments and methods employed ; scientific maritime 

 expeditions, which in number and magnitude have exceeded 

 those of any other century, and which have been carefully 

 equipped at the expense of their respective Governments, 

 and favoured by the happy choice both of the commanders 

 and of the observers who have accompanied them ; and 

 various expeditions by land, which having penetrated far 

 into the interior of continents, have been able to elucidate 

 the phenomena of terrestrial magnetism, and to establish a 

 large number of fixed stations, situated in both hemispheres 

 in corresponding north and south latitudes, and often in 

 almost opposite longitudes. These observatories, which are 

 both magnetic and meteorological, form as it were a net- 

 work over the earth's surface. By means of the ingenious 

 combination of the observations which have been published 

 at the national expense in Russia and England, important 

 and unexpected results have been obtained. Tlie establish- 



