at the Island of Balta, and at Woolwich Common. 429 
to this tripod, the clock-case is adjusted to horizontality and ver- 
ticality, after the manner of portable transit instruments; two 
spirit-levels which are fixed within the clock-case at right angles 
in the same horizontal plane, enabling us to ascertain when the 
adjustment is complete. 
In Balta the three feet on which the clock and its case thus 
rested, were placed upon three separate portions of rock which 
were chamfered down till they were nearly in one horizontal 
plane; the position of the clock was then easily rectified by 
means of the adjusting screws. At Woolwich Common those 
feet are placed upon the heads of three separate piles which stand 
detached from the floor of the Observatory. In Balta a suitable 
piece of rock was selected as the pedestal of the éranstt instru- 
ment (which is a portable one by Troughton): the top of the 
rock was properly levelled; holes were drilled to receive fused 
lead, on which, after it had cooled, the feet of the transit instru- 
ment were placed. A wooden frame was placed round the piece 
of rock, to keep the observer from accidentally pressing against 
it during the observation. At Woolwich Common the transit 
instrument is placed upon a piece of stone thirteen inches square 
and six deep, which is half embedded in a strong prismatic box 
of sand, that rests upon piles. This, which I recommended as 
an appropriate pedestal for a transit instrument, has been found 
‘ to answer remarkably well; the instrument, when properly ad- 
justed, having preserved its level accurately for months in suc- 
cession *, 
These particulars are premised from a conviction that the 
public cannot be expected to place any confidence in the results 
of an experiment, unless the means taken to guard against the 
probable errors are fairly explained. Under this persuasion I 
shall, as I proceed, enter into minutiz, where their omission 
might occasion doubts; and may here mention, while speaking 
of the clock, that the actuating weight was uever permitted to 
descend lower than within two inches of the top of the pendulum 
ball ; lest a nearer approximation of two such masses of brass, 
- might produce irregularity (however slight) by their mutual at- 
traction. 
The small island of Balta, upon which M. Biot, Captain Colby, 
and myself fixed, after mature deliberation, as by far the most 
convenient on the whole, for landing our apparatus and carrying 
* Such a box may be readily made to take to pieces, and join together 
again by screws; and will then serve to accompany a portable transit jn- 
strument from place to place, as it may be needed. But for a permanent 
observatory, it might be well to have the sand contained in a case of bricks. 
The instrument would then, I am persuaded, be less affected by any exter- 
nal impressions, than if it stood on a solid block of stone. 
on 
