MAGNETIC APPARATUS. 151 



Hansteen, in order to ascertain the force with which a magnetic 

 needle is urged towards its position of equilibrium at different 

 parts of the earth and at different times at the same place. The 

 method adopted in these observations, however, served merely to 

 compare the force acting at one time and place with that acting at 

 another. The great advance made by Gauss consisted in show- 

 ing how to eliminate the effect due to the particular magnet used, 

 and so, instead of merely comparing one place with another, to 

 express the absolute intensity of magnetic force at a given place 

 and time in terms which do not involve reference to any physical 

 magnitudes except the fundamental units of length, mass, and 

 time. Gauss also contributed very greatly to increase the prac- 

 tical accuracy of magnetical observations by the improvements he 

 introduced in the construction of instruments ; and especially by 

 the substitution (first suggested by Poggendorff in 1826) of straight 

 divided scales, observed by reflection in a mirror attached to the 

 magnet, for graduated circles,, whenever small changes of angular 

 position were to be measured. 



In 1836 the Gottingen Magnetic Association was established 

 for the purpose of making, periodically, simultaneous observations 

 of the magnetic elements in different parts of the world, according 

 to a common plan. The yearly volumes of " Results " of this 

 Association, published by Gauss and Weber from 1837 to 1842, 

 will long be consulted, not only for the stores of accurate obser- 

 vations recorded in them, but also for the details which they con- 

 tain of the most perfect methods yet introduced for the measure- 

 ment of any kind of physical phenomena. A few years later, 

 chiefly through the exertions of Sir Edward Sabine, Magnetic 

 Observatories, for observations according to the Gottingen system, 

 were established by the Government in several of the British 

 Colonies. 



In conclusion, we must not omit to mention the valuable 

 labours of Lamont, ot Munich, and Lloyd, of Dublin, in the 

 construction of magnetic instruments ; nor to refer to the method 



