304 Description of the Solar Index. 
of the spirit level on the arm LM into the middle of the opening, 
and fix it in this position by again tightening the screw F. Now 
give the compass a slow motion upon the tripod, and observe if 
this bubble maintain its position ; if so, this adjustment is properly 
made; if not, repeat the latter part of the process; and in turn- 
ing the compass, move the quadrant also a trifle upon the equato- 
rial axis till the adjustment be well made. Fix now the 0 of the 
vernier to a distance from the 0 on the middle of the quadrant, 
equal to the declination of the sun at noon at the given place on 
the day of observation, above this 0, if the declination be north, 
but below it if it be south; turn the quadrant and compass so as 
to allow the sun’s rays to pass through the lens and fall upon the 
piece O, when the sun is on the meridian; and adjust this piece 
in such a manner that the focal area of the sun shall overlap the 
area of the aperture in the piece, and form a fine, even ring about 
its edge. It will be well to test this adjustment on several days. 
When this has been done and the piece O firmly fastened by its 
screw, no further adjustments will be requisite except for different 
latitudes and declinations; and these can be easily effected by the 
surveyor or engineer in a very few minutes. The correct adjust- 
ments, indeed, of the base A, the quadrant, and level upon the 
arm LM, the tube with regard to the horizontal axis B, the lens, 
and the piece O, all belong to the manufacturer. How the piece 
O is adjusted I have already described. I will soon show in 
what manner the surveyor or engineer may make or test for him- 
self these other adjustments. At present I will state how the in- 
strument should be used for the purpose designated. 
On any day when the sun shines, let the compass with the 
index attached be leveled, and the adjustments made for the lat- 
itude of the place, and the sun’s declination for the hour; turn 
the compass with the index till the sun shines through the lens 
and formsa ring about the aperture in O; and the north and south 
points on the horizon of the compass will be in the true meridian, 
and the variation of the needle be seen at once. If observations 
at different times in the forenoon and afternoon give the same 
variations* at one and the same place, it is proof that all the ad- 
justments have been well made. 
In a survey of no great extent no new adjustments will be 
necessary for change of latitude; and if the ¢ime occupied in the 
* Or nearly ; see first note following. 
