At long range it is also necessary to consider the
motion of the vessel across the line of fire. Suppose this to be at
the rate of six knots, and the gun is elevated 15 deg., the time of flight
would be by the Tables, 18 sec., while the deviation arising from this
cause would be upwards of 60 yards. It is therefore of great
importance that the Captain of the gun shall be carefully instructed
in making this adjustment of the eye-piece.

408. ELEVATING SCREW.--To obtain readily the changes of elevation
necessary in the use of rifled cannon, the heavier calibres are made
with very small preponderance, and are supplied with an elevating
screw which is attached to the carriage at the lower end, while the
nut is connected with the cascabel of the gun. Both screw and nut
admit of movements by which the screw can take any position required
in the various degrees of elevation. The parts should be allowed a
certain amount of play; if binding is prevented, it is believed that
the evident advantages of the screw may always be obtained.

409. RANGES AND TIME OF FLIGHT.--So far as ascertained, are contained
in Table VII., Appendix B.

410. PRECAUTIONS TO BE OBSERVED.--In the use of these rifled cannon,
it is of the utmost importance that all the directions relative to the
lubrication of the shell, its being close home, charge and kind of
powder used, and lining of the shells, shall be carefully observed.

Many premature explosions of shells having taken place in these guns,
which are attributed to various causes, such as,--defects of metal,
porosity, faulty fuzes, concussion and friction of the powder within
the shell,--it is ordered that, on the occurrence of a premature
explosion or rupture of a shell within the gun, it shall be
immediately washed out and a careful examination made of the interior
of the bore, by the mirror and by taking impressions in wax (_see_
Mode of Taking Impressions, p. 16, Part III.), and all the
circumstances of the case reported to the Bureau, specifying the kind
and calibre of the shells, kind of fuzes, the charge and kind of
powder, with its manufacturer's name: and,

Were the shells filled completely, and with what kind of powder?
Were the shells coated inside with any kind of composition?
Are any cracks or marks of scoring visible in the bores?

In rifled cannon, cracks or injuries produced by firing, or the
rupture of shells, are to be sought for--thus,

1. Around and in rear of t