1831.] Determination of the Latitude of Kartigora in Kachar. 301 
II, Determination of the Latitude of Kartigora in Kachar. By 
Lieut. T. Fisher, Dep. Asst. Qr. Mr. Gen. 
The full account of the double altitude instrument, given in the memoirs of the 
Astronomical Society of London, with the notice in the 1st No. of Gleanings in 
Science of the particular instrument with which the annexed observations were 
made, preclude the necessity of again describing it. 
It may however be as well to remind the reader, that this instrument is intended 
to supersede the employment of a level as a zero, and consists of two separate ver- 
tical circles, to be used contemporaneously (or very nearly so), in the measurement 
of the angle formed by a heavenly body, and its image reflected from a trough of 
mercury or other horizontal surface. Thus, while the telescope of one circle is 
pointed to a star, that of the other is directed to its image, and after their separate 
readings have been registered, the positions are reversed, the elevated telescope 
being depressed to the image, and the depressed telescope elevated to the object ; and 
new readings obtained, completing the measurement of the angle on each circle, 
compensating for the error in collimation, and lor any change in the position of 
the zero points, which may have taken place in the interval between the two obser- 
vations : either the corresponding times or azimuths must of course be noted 
with the altitudes, in order to reduce them to the meridian. 
To save the labour of this last reduction, the observations contained in Tab. No. 
1, were made so near the meridian, as to render it unnecessary ; the contact of the 
wire and object being completed in the one telescope about eight or ten seconds 
before the transit, and that in the other about the same time after it. The reversed 
observations in this case are necessarily made on a subsequent night, which is a 
disadvantage, as there is a chance that in the intermediate time unequal changes 
take place in the positions of the micrometers ; this risque, howc\er, does not 
appear from experience to be very great, provided the interval be not considerable. 
It may also be easily detected by noting occasionally the differences of each pair of 
micrometers when directed to a fixed mark, the position of the instrument being 
at the same time verified by means of the telescope under the horizontal ciicle. 
When single altitudes only are taken, the level on one of the telescopes is used 
83 a zero, the telescope being directed to a fixed mark, and the reading on its 
c >rcle being carefully noted, with a view to detect any changes that may take place 
the position, either of the instrument bodily, or of the micrometers several 1\ . 
The advantages of the instrument are sufficiently obvious ; besides enabling us 
10 dispense with the level, and thereby cutting off all its uncertainties, we have a 
c °mpl e te set of observations on two circles, independent of each other, except as 
re S‘rds the small differences that depend on any change in the zero points, and 
c °naeqnently each set may be estimated at twice the value of one made with a single 
altitude instrument. 
fi‘ e only objection to which the construction appears liable, is the shortness of 
| l( transverse axis, which it is difficult to keep level, as very small inequalities in 
lf P' v °ts produce sensible effects on it, and the circle is consequently liable 
0 * )e thrown out of the vertical ; which in measurements made near the zenith, or 
a considerable distance from the meridian, may be products c of • ome error. 
r,ia ically, however, this disadvantage has not been found so great, as might have 
.7*° ex Pected from theory ; and it should be observed, that the error arising from 
cause may be compensated, where the latitude only is sought, by observing pairs 
which have nearly equal meridian altitudes, but are on opposite sides ot 
the zenith. 
