4 Description of a new Transit Instrument. 
Fig. 4 is the riding level, and is placed upon the axis in the 
Ys, and adjusted by means of the screw S. Fig. 5 represents 
the stand, telescope, and level, displaying the position in which 
they are used. Fig. 6, for placing the transit in the meridian. 
Of the verifications of this instrument two are common to 
every construction ; one only is peculiar to this. The line of 
collimation is adjusted by looking direct at some distant small 
point (the cover being turned over the lateral aperture), and 
turning the telescope gradually round on its axis, and moving 
the screw of the wires, if necessary, till the spot is in every po- 
sition covered by the intersection of the wires. The axis is 
brought to an horizontal position, and the level is in the same 
mode adjusted by reversing the telescope or level, and correcting 
half the error by the level screw, and half by the vertical screw 
of the Y in the usual mode. 
The verification peculiar to this instrument is that of the 
mirror, and perhaps the best mode of doing this is by the pole 
star when nearly in an eastern or western position from the pole; 
its motion in azimuth is then so slow as to give ample time for 
the adjustment. Bring the pole star to the vertical wire (the line 
of collimation having been previously adjusted) ; then reverse the 
telescope in its Ys ; and if the star is still on the wire, the mirror 
is in adjustment : if net, correct half the difference by the mirror- 
screw, and half by the horizontal of the Y till the error vanishes. 
This adjustment may also be performed by setting up a board 
with two parallel perpendicular lines drawn on it distant from 
each other, exactly the space between the positions of the mirror 
when the telescope is reversed in its ¥s._ If the vertical wire he 
brought to cover one of the marks, and on being reversed the 
wire covers the other mark, it is right. If not, the error, must 
be made to vanish by cerrecting it half and half as before di- 
rected for the adjustment by the pole star. 
The following method of placing the instrument correctly in 
the meridian, is equal if not superior to any that has yet been 
devised*, 
Let Z, fig. 6, be the zenith; P, the pole; HO, the horizon; 
ZPI, the meridian circle; ZK, a circle of altitude distant from 
the meridian by a small quantity IK (suppose adegree) ; 1,2,3,4, 
the diurnal circle of the pole star, whose radius is 1° 45” nearly ; 
and let the altitude of the pole be 51° 30’.. Then when the pole 
star is on the northern meridian its altitude 31 will be 49° 45’, 
and its zenith distance Z3=40° 15’; and ACD be a part of 
the diurnal arch of a star whose polar distance is 46° 30’, and 
N. meridian altitude 5°. 
* It had been invented hy me many years ago, but has not, as far as I 
know, been as yet published. I theretore give it here, as not inapplicable 
to the subject of this p:per. 
Now, 
