ens he can at once practically
find; and the basis for the determination of the time is the
constantly but _very irregularly_ varying zenith distance of the sun.

At sea the observation of the altitude of a celestial body is the only
method available for finding local time; but the perfection which has
been attained in the construction of the sextant enables the sailor to
reckon on an accuracy of seconds. Certain precautions have, however,
to be taken. The observations must not be made within a couple of
hours of noon, on account of the slow rate of change at that time, nor
too near the horizon, on account of the uncertain refractions there;
and the same restrictions must be observed in using a portable dial.

To compare roughly the accuracy of the fixed and the portable dials,
let us take a mean position in Great Britain, say 54 deg. lat., and a
mean declination when the sun is in the equator. It will rise at 6
o'clock, and at noon have an altitude of 36 deg.,--that is, the portable
dial will indicate an average change of one-tenth of a degree in each
minute, or two and a half times slower than the fixed dial. The vertical
motion of the sun increases, however, nearer the horizon, but even there
it will be only one-eighth of a degree each minute, or half the rate of
the fixed dial, which goes on at nearly the same speed throughout the
day.

Portable dials are also much more restricted in the range of latitude
for which they are available, and they should not be used more than 4
or 5 m. north or south of the place for which they were constructed.

We shall briefly describe two portable dials which were in actual use.

_Dial on a Cylinder._--A hollow cylinder of metal (fig. 7), 4 or 5 in.
high, and about an inch in diameter, has a lid which admits of
tolerably easy rotation. A hole in the lid receives the style shaped
somewhat like a bayonet; and the straight part of the style, which, on
account of the two bends, is lower than the lid, projects horizontally
out from the cylinder to a distance of 1 or 1 1/2 in. When not in use the
style would be taken out and placed inside the cylinder.

A horizontal circle is traced on the cylinder opposite the projecting
style, and this circle is divided into 36 approximately equidistant
intervals.[2] These intervals represent spaces of time, and to each
division is assigned a date, so that each month has three dat