180 Lieut . Foster's comparison of the diurnal changes 
ground, at the distance of about eight feet from the middle 
of the box, in the direction of the magnetic meridian : when 
the needle was at rest in its natural direction, a fine thread 
of light reflected from its end, was bisected by the vertical 
Wire in the telescope ; the telescope having a lateral sliding 
motion for the purpose of accomplishing this adjustment. 
In making a set of these observations, the following mode 
was pursued : the needle being suspended horizontally, the 
adjustment of the telescope above described was first com- 
pleted, after which, the needle was made to vibrate at the 
commencement, in an arc of 60 degrees, by the contrivance 
already alluded to ; the time at which the reflected thread of 
light passed the wire in the telescope, was noted by means 
of a chronometer, and also at every tenth vibration follow- 
ing, until one hundred were completed : the needle was 
then removed from the box, and placed on its axis in the 
dipping apparatus ; the time of its performing one hundred 
vibrations (commencing as before in an arc of 60 degrees) 
was in like manner noted ; the passage of the central point 
in this case being determined by means of a lens, fixed over 
that part of the vertical circle to which the needle pointed, 
when freely supported on its axis and at rest. In this way 
all the results in the following Table have been obtained ; it 
may not, however, be unimportant to state, that although 
the needle, in each of its different positions, always vibrated 
in the same arc at the commencement, viz. 60 degrees ; yet 
the terminal arc, in either position, generally varied. 
The Table is divided into two parts ; the first contains the 
observations on the times of vibration of the needle in its 
horizontal position ; and the second, those on it when used as 
