Description of the Solar Index. 305 
survey does not extend beyond two or three hours, no alteration 
of adjustment will in general be requisite for change of de- 
clination. 
An hour or two about the middle of the forenoon and afternoon 
are the times most favorable to the greatest degree of accuracy in 
these observations. When the sun is near the horizon, the re- 
fraction of his rays, may produce an error of several minutes in 
the result; and when he is near the meridian, a small error in 
any of the adjustments is the more likely to affect the accuracy 
of the result; but less in both these cases than can be estimated 
by compasses in general use. 
It would be very convenient to havea level placed upon the 
base A, underneath and parallel with the horizontal axis B; 
though it can be dispensed with. 
A compass with a horizon accurately graduated and furnished 
with a vernier capable of measuring an arc of one or two minutes, 
will, with this index attached, afford the means of making very 
accurate surveys. Let all parts of the support of the compass be 
strong and well made; and let not any little difficulties which 
always attend the first trials of a new instrument, create doubts 
as to its complete success. Fora very little practice will show 
the exceeding simplicity of its structure, and the facility and ac- 
curacy with which it operates. 
And I wish here to impress upon surveyors and engineers the 
importance of noting all their courses from the true meridian ; 
and of adding, in all their reports of surveys, the variation of the 
needle. JI know this is already the practice of certain individuals 
among them; but were the practice general it would put an end . 
toa multitude of mistakes, and arrest in some portions of our 
country a large amount of litigation, which has arisen from the 
difficulty of finding the fluctuations of variation in individual 
surveys. ‘These fluctuations are mainly owing* to the proximity 
of iron ores, and occur much more frequently in surveys than I 
formerly apprehended.t 
*T say mainly ; the influence of the sun upon the needle causes its declination 
from the meridian to be greater by 13! to 15/ at 3 o’clock p.m. than at 8 o’clock 
A.M., in the summer ; and 8! to 10! greater in winter. 
t And I have no doubt that cases like the following, reported by Capt. Scoresby, 
are frequent at sea, and if not guarded against produce serious disasters. In sail- 
ing down the British Channel, he observed the variation of the needle to be 2° or 
3° greater than in sailing upit in the same ship, which he attributed to the improper 
lading of the ship. 
