208 Description of the Instruments 
apparently observe no law. 2. The periodical varia¬ 
tions, dependent upon the sun’s hour angle. 3. The 
secular variations, which are either slowly progres¬ 
sive, or else return to their former values, at periods 
of great and unknown magnitude. To supply a complete 
knowledge of these desiderata, magnetical observatories 
have been established at stations widely scattered over 
the earth’s surface.—But, before proceeding to describe 
the instruments employed in these investigations, it may, 
perhaps, be desirable (for the information of those persons 
who may not already be conversant with the subject) to 
state more fully what those elements are which have 
been taken as the immediate object of research. 
If a vertical plane be conceived to pass through the 
line of direction of a freely suspended magnetic needle, 
then, upon determining the inclination of the needle to 
the horizon in that plane, as well as the angle which the 
plane itself forms with the meridian, the direction of the 
magnetic force becomes known. If, further, a number 
can be ascertained which expresses the ratio of the inten¬ 
sity of the force to some established unit, it is evident 
that the force is completely determined. It is found 
convenient for practical purposes, that the intensity 
should be observed in two separate portions, one acting 
in a horizontal, the other in a vertical plane ; as any force 
may be supposed to be composed of two or more forces 
acting in different directions. These two portions or 
components may be afterwards substituted for the total 
intensity and inclination, while, at the same time, their 
changes can be determined by actual observation with 
far greater precision. 
The first element on which the measure of the terrestrial 
magnetic force is based has been already named as the 
Declination, and it represents the angle which the mag¬ 
netic meridian at any given place forms with the true men- 
