LIEUT. -COLON EL SABINE ON TERRESTRIAL MAGNETISM. 
123 
6. only were employed : the deductions by No. 6. being- in all such cases dependent 
on the determinations by all the other needles at the base stations visited immediately 
before and after. When the force at the intermediate place has appeared by No. 6. to 
be nearly the same, whether derived from the base station preceding, or the one fol- 
lowing it, the mean result has been considered to have an independent value, and 
has been employed accordingly. When otherwise, the observations have been 
entered in the table, but no deductions have been made from them. 
The values of the horizontal force which are contained in the final column of the 
general table are expressed in absolute measure, referring to the units prescribed in 
the magnetic Instructions which have received the sanction of the Royal Society*. 
Experiments made by Lieutenants Lefroy and Riddell in the Royal Military Repo- 
sitory at Woolwich in May 1842, with magnets of four inches in length, gave 3’72 as 
the approximate value of the horizontal force at Woolwich at that period, agreeably 
to the following memorandum : — 
“The number 3*/2, given as the approximate value of the horizontal intensity at 
Woolwich, expressed in English units of feet, seconds, and grains, was determined 
from experiments of deflection and vibration made with one of Weber’s transport- 
able magnetometers. 
“The experiments of deflection were made after the method of M. Gauss, with the 
axis of the deflecting magnet perpendicular to the magnetic meridian, the angles 
being measured on the scale fixed over the reading telescope. The deflecting and 
suspended magnets were of the same dimensions, about four inches in length and 
four-tenths in diameter. 
7YL 
“The values of were calculated from the several pairs of observations by the 
formula 
m r ,b tan u' — r b tan u 
X 2 (r n — r 2 ) 
“ The partial results, distances, and angles of deflection are given in the accom- 
panying abstracts. 
“The moment of inertia of the vibrating magnet was determined by observing a 
* “The number obtained for the force of the earth’s magnetism expresses the ratio which that force bears 
to the unit of force, the unit of force being that which, acting on the unit of mass, through the unit of time, 
generates in it the unit of velocity. These units are entirely arbitrary ; but for the sake of convenience in com- 
parison, it is desirable that they should be the same in all the observations which shall be made according to 
this system. For the unit of mass, then, we may take a grain ; for the unit of time a second ; and, if a foot be 
taken as the unit of space, the unit of velocity will be that of one foot per second. 
“ As the magnetic force operates effectively only on the free or uncombined elements of the magnetic fluid, we 
are to understand by the earth’s magnetic force, its action on the elementary unit of free magnetism ; and we 
must take for that unit the quantity of free magnetism, which, acting on another equal quantity at the unit of 
distance, exerts an effect equal to the unit of force already defined .” — Royal Society, Report of the Committee of 
Physics, 8(C., approved by the President and Council, 1840, pp. 21, 22. 
MDCCCXLIII. 
S 
