THE RIVER THAMES AT LONDON BRIDGE AND THE SEA. 
195 
difference in weight of those parts of the telescope outside the collar. I found 
this difficulty in my instrument : to remedy it, I measured off from one of the 
resting points half the distance between the two Ys on the collars, and sus- 
pended the telescope by a fine wire from this point, which was the proper 
centre of the telescope. I found the eye-end with the tube close in to be more 
than four ounces heavier than the object-end. To remedy this, I caused a 
thick ring of lead of the above weight to be placed inside the tube near the 
object-end, by which the telescope was balanced; and I found it, when ad- 
justed, to reverse without differing a quarter of a degree. 
Distinct vision is certainly desirable, but not so absolutely requisite as that 
there shall be no parallax of the wires. The best way to avoid this, is, after 
adjusting for distinct vision, to move the eye as far as the hole in the eye-piece 
will admit of, and observe if the wires have any motion over the object or 
vane : if so, it must be remedied by sliding the eye-tube in or out, until the 
objects appear motionless. 
Mirage. 
I have found that the tremulous motion or jumping in the air, termed as 
above, appears not to be caused so much by evaporation as probably by some 
oscillation in the particles of light : for I have remarked, when the sun shines 
brightly and is occasionally obscured by clouds, that while the sun is opt, the 
tremor is so great as to prevent the possibility of making a correct observa- 
tion ; yet the moment the sun is obscured, the intermediate space between the 
instrument and object (provided the sun is obscured so as to cast a shadow 
the whole distance) will be immediately perfectly tranquil ; and again, at the 
instant of the sun’s appearance, the same tremor will be observed. 
I have found this motion to be exactly equal above and below any object ; 
for upon placing the wire of the telescope one half the distance between the 
extreme oscillations, whenever the sun is obscured, the wire will be found to 
bisect the object. 
Description of the Observation-Book. 
Each page of the book will be found to contain sixteen columns ; the first 
and ninth contain the numbers of each staff or picket ; the second and eleventh 
the mean from the rough book of the four observations at each staff ; the third 
2 c 2 
