Plane trigonometry, by S.L. Loney.

CHAPTER XXXI.

ERRORS OF OBSERVATION.
391. WE have up to the present assumed that it is
possible to observe any angles perfectly accurately. In
practice this is by no means the case. Our observations
are liable to two classes of errors, one due to the instruments themselves, which are hardly ever in perfect adjustment, and the other class due to mistakes on the part of
the observer.
392. An error in any of our observations will clearly,
in general, cause an error in the value of any quantity
calculated from that observation. For example, if in Art.
192 there be a small error in the value of a, there will be
a consequent error in the value of x which, as we see
from the result of that article, depends on a.
393. The importance of an error in a length depends,
in general, upon its ratio to that length. For example in
measuring a piece of wood, about six feet long, a mistake
of one inch would be a very serious error; in measuring a
mile racecourse a mistake of one inch would be not worth