211 
employed in the Observatory. 
the force acting in the horizontal plane, tending to direct 
the needle and causing it to oscillate. Hence it can be 
demonstrated, that HA, the horizontal force, will be 
equal to R the total force x cos. I, the inclination, or 
HA=R x cos. I. 
It is by this method that the magnetic intensity has 
hitherto been observed, and, as the apparatus is very 
portable, it is practicable under almost all circumstances. 
Professor Hansteen, to whom the science of terrestrial 
magnetism is so much indebted, has, from numerous 
observations made in all parts of the globe, projected 
curves passing through all places where the intensity is 
equal, to which he has given the name of “ isodynamic ” 
or those of equal force. 
We will now proceed to examine the instruments by 
which the before-mentioned elements are to be observed; 
and as they are novel in form, and capable of a refine¬ 
ment in observation not hitherto attained, it may be 
interesting to give a detailed account of their construc¬ 
tion. 
The first is called the “ Declination Magnetometer.” 
It is employed first in observations of the absolute de¬ 
clination ; secondly, of the variations of the declination ; 
and thirdly, of the absolute intensity. 
The principal parts of this instrument consist of a 
highly magnetised needle, or bar of steel, in the form of 
a rectangular parallelopipedon, 15 inches in length, g of 
an inch in breadth, and \ of an inch in thickness, sus¬ 
pended by fibres of untwisted raw silk, and enclosed 
in a circular box to protect it from currents of air. 
A finely divided scale of glass is fixed upon one 
end of the needle, and on the opposite an achromatic 
lens. The scale being adjusted to the focus of the lens, it 
is evident that the whole apparatus forms a moving 
collimator; and its changes of position can be read off 
r 2 
