ation. If closely kept up, it will prove to be of
utility, and will in some degree reward the trouble of keeping a regular
record. For purely scientific objects much more nicety and detail are
required.

* * * * *

HESITATION is sometimes felt by young seamen, at first using the vernier
of a barometer, for want of some such familiar explanation as the
following:--

The general principle of this moveable dividing scale is, that the total
number of the smallest spaces or subdivisions of the vernier are made
equal, taken altogether, to one less than that number of the smallest
spaces in an equal length of the fixed scale.

For example: ten spaces on the vernier being made equal to nine on the
scale, each vernier space is one tenth less than a scale space; and if
the first line or division of the vernier agree exactly with any line of
the scale, the next line of the vernier must be one tenth of a tenth (or
one hundredth) of an inch from agreement with the next _scale_ division;
the following vernier line must be two hundredths out, and so on:
therefore, the number of such differences (from the next tenth on the
scale) at which a vernier line agrees with a scale line, when set, is
the number of hundredths to be added to the said tenth; (in a common
barometer, reading only to hundredths of an inch).

The vernier of a barometer reading to thousandths of an inch, is on a
similar principle, though differently divided. In this application of
it, generally, twenty-five vernier spaces equal twenty-four of the scale
spaces, which are each half a tenth, or five hundredths of an inch;
therefore, the difference between one of the vernier and one of the
scale is two-tenths of a hundredth, or two thousandths of an inch
[25).050(.002].

This is the usual graduation of scientific barometers; but for ordinary
purposes, as weather-glasses, a division, or reading, to the hundredth
of an inch is sufficient.

When set properly, the vernier straight edge, the top of the mercury,
and the observer's eye, should be on the same level; the edge (or
pointer) just _touching_[36] the middle and uppermost point of the
column.

Great care should be taken to look thus square, or at right angles to
the scale.

Light, or something white, at the _back_ of the tube, assists in
accurately setting the vernier, and may be shifted about to aid in
reading off.

* * *

THE ANEROID has been recommended