the Bible, unless its predictions can be calculated by algebra.
It may do such persons good, or at least prevent them from doing others
harm, to take a cursory view of the errors of astronomers; errors
necessary as well as accidental.

Sir John Herschel, than whom none has a better right to speak on this
subject, and whose devotion to that noble science precludes all
supposition of prejudice against it, devotes a chapter to _The Errors of
Astronomy_,[333] which he classifies and enumerates:

"I. External causes of error, comprehending such as depend on
external uncontrollable circumstances; such as fluctuations of
weather, which disturb the amount of refraction from its
tabulated value, and being reducible to no fixed laws, induce
uncertainty to the amount of their own possible magnitude.

"II. Errors of observation; such as arise for instance from
inexpertness, defective vision, slowness in seizing the exact
instant of the occurrence of a phenomenon, or precipitancy in
anticipating it; from atmospheric indistinctness, insufficient
optical power in the instrument, and the like.

"III. The third, and by far the most numerous class of errors,
arise from causes which may be deemed instrumental, and which
may be divided into two classes.

"The first arises from an instrument not being what it
professes to be, which is _error of workmanship_. Thus if an
axis or pivot, instead of being as it ought, exactly
cylindrical, be slightly flattened or elliptical--if it be not
exactly concentric with the circle which it carries--if this
circle so called be in reality not exactly circular--or not in
one plane--if its divisions, intended to be precisely
equidistant, shall be in reality at unequal intervals--_and a
hundred other things of the same sort_.

"The other subdivision of instrumental errors comprehends such
as arise from an instrument not being placed in the position
it ought to have; and from those of its parts which are made
purposely movable not being properly disposed, _inter se_.
These are _errors of adjustment_. Some are unavoidable, as
they arise from a general unsteadiness of the soil or building
in which the instruments are placed.[334] Others again are
consequences of imperfect workmanship; as when an instrument,
once well adjusted, will not remain so. But t