THE RIVER THAMES AT LONDON BRIDGE AND THE SEA. 
169 
not more than three feet distance from the station-staff, several lenses were 
made of different focal distances, as sixty, thirty, fifteen, eight feet, and thirty 
inches ; which, being applied to the object end of the telescope, converts it, in 
fact, to a microscope. 
To the lower part of the telescope, within the collars, are affixed the cocks, 
into which fits the tube protecting the level : one of these cocks gives a ver- 
tical, and the other a horizontal motion to the tube, in order to place the level 
parallel to the axis of the telescope. 
When the instrument was first made, these motions were effected by means 
of endless screws ; but I found it so difficult (almost impossible) to keep a 
delicate level in adjustment by this mode, that I substituted the old fashion of 
capstan-headed pushing screws. 
To the upper part of the telescope are attached (outside either points of sup- 
port) cocks* or braces, carrying a swinging level, having, as well as the cocks, 
separate adjustments. 
This additional level was intended as a check to the lower level, and to 
detect any occasional variation in the figure of the tube itself. 
The glass bubbles themselves were placed in the tubes at first with paper, 
and wedged at either end with small pieces of wood ; but the wedges are 
liable to distort the bubble itself, and after some time get loose in the tube ; 
and the level alters in its position, and is never to be depended on. I found it 
better to push the level into its proper place in the tube, not tightly, and with 
paper underneath, taking care that the paper touches the middle part, and 
then filling up the two ends with plaster of Paris. 
To the upper part of the telescope, near the eye-piece, is fixed a small level, 
adjusted to the horizontal wire of the telescope, and by the assistance of which 
the same surface of the large level is used at each observation. There is also 
another small level at the other end of the telescope, to adjust the vertical 
wire by, but of less use than the former. 
On the upper limb, near one of the Ys, is fixed a small thermometer, with 
the bubble inclining downwards, at an angle of about 10°, the use of which 
will be explained hereafter. 
The large stand of this instrument is made with a solid top of African oak. 
* These cocks are not shown in the Plate. 
MDCCCXXXI. 
Z 
