Description of a new Transit Instrument. ; 3 
I am convinced that transit instruments of the largest size 
might with very great advantage be constructed on this principle. 
It is true that where very much light is wanted, as in obser- 
vations of stars in the day-time, the loss by reflection will be some 
disadvantage ; but the loss of light from an unsilvered mirror is 
very small; and the convenient and simple form of the instru- 
ment, by its lightness, less subject to flexure; by its position 
much less liable to errors from unequal change of temperature 5 
and so extremely commodious in its use; present advantages of 
a very important nature, and such as might introduce it into the 
most extensive observatories with profit. The advantages may 
perhaps be even greater than in small transit instruments. When 
in the present construction the telescope is long, as its whole 
weight rests on the most disadvantageous point of the axis, this 
is of necessity made very large towards its centre to avoid flexure, 
the whole instrument so heavy that an additional apparatus of 
counterpoise must be added to the pivots, lest they should wear 
away the Ys, and the reversing the instrument becomes a work 
of some difficulty and danger: whereas in the construction pro- 
posed, the small comparative weight of the object- and eye- 
glasses lies very near the pivots, and the middle of the tube is the 
lightest part of it. The operation of reversing is performed with 
great ease, both from the form and lightness of the instrument ; 
and it may be added, that the comparative facility of observa- 
tiofis is of greater advantage the larger the instrument. 
Having so far explained the general principles of this transit 
telescope, which was executed very much to my satisfaction by 
Mr. Thomas Jones, it will be proper to describe more particularly 
its construction. 
The transit stand is represented by either fig. 1 or 2 (P1.I). The 
first is constructed for being placed or fixed on a vertical surface 5 
the second for a horizontal one, A,B; in both are the Ys, or 
supports for receiving the axis of the telescope. The end A in- 
tended to be always next the eye is furnished with both the hori- 
zontal and vertical adjustments, such as are usual to transit in- 
struments. The telescope axis is represented by fig. 3. The eye 
end being at C, the object end at D turns round in the Ys AB 
upon its cylinders 7, 2; the screw head r, at the object end D, is 
for the purpose of adjusting the parallel glass. The telescope 
is adjusted to distinct vision by means of the head or knob at Fe 
The wires of the telescope are adjusted by means of the four 
capstan head-serews at the eye end ¢,f. The circle R is divided 
on the surface next to the eye. The eye-tube has a sliding mo- 
tion for viewing the wires distinctly; the short ptece of tube 
at the object end turns round on the telescope tube, and serves 
as a cover for the object-glass. 
é A2 Fig. 
