478 The Account of a 
were in the same right line, as discovered by the application of 
this board to their heads. 
The method of determining the angles which the measured 
lines made with the plane of the horizon was as follows. 
After the hypotenuse was measured, the transit-instrument 
with its boards were placed on the picket, and the leveliing- 
screws moved as before described, if the axis did not happen to 
be horizontal. The cross board, upon which a black line was 
drawn whose breadth was about twice the apparent thickness of 
the micrometer-wire, and its distance from the bottom of it equal 
to that of the axis of the instrument from the register-head. 
Was placed on another picket in the hypotenuse, having the 
brass head which had been before fixed on it still remaining. 
The telescope was then made horizontal, the index of the mi- 
crometer being placed to the zero on its circle, and the wire of 
the microscope set to bisect that dot on the arch which was 
nearest to the centre of the field. After this, the telescope 
was moved in the vertical by the finger-screw, till another 
dot was bisected, at the same time that the line upon the cross 
board appeared in the glass, by which the angle that the 
instrument had described on its axis, was measured in half 
degrees. The remaining part of the angle, or rather the frac- 
tional part of an half degree, was measured by the micro- 
meter, the wire of which was brought from the centre of the 
glass to bisect the black line, and was either added to, or sub- 
tracted from, the former quantity, as the angle described by 
the telescope fell short of, or exceeded, that formed by the 
hypotenuse and the plane of the horizon. 
By this method, all the angles of elevation and depression 
were taken. And we consider it as probable that they are 
