air or by a direct steam impulse
arrangement these weapons are started on their course and are
directed, and then the running is taken up by their own engines
operating on screw propellers, driven by a magazine of compressed
air contained in the body of the torpedo. Means are also provided to
maintain the designed level below the water surface. The torpedo may
either be projected from the war ship itself or from one of those
launches which owe their origin to our member, Mr. John Isaac
Thornycroft, who first demonstrated the feasibility of that which was
previously considered to be impossible, viz., the obtaining a speed
of twenty miles and over from a vessel not more than 80 feet long.
Experiments have been carried on in the United States by Captain
Ericsson to dispense with the internal machinery of the torpedo, and
to rely for its traverse through the water upon the original impulse
given to it by a breech-loading gun, carried at the requisite depth
below the water level in a torpedo boat. This gun, having a feeble
charge of powder at a low gravimetric density, fires the torpedo, and,
it is said, succeeds in sending it many yards, and with a sufficient
terminal velocity to explode the charge by impact. Also, in the United
States, experiments have been made with a compressed air gun of
40 feet in length and 4 inches in diameter (probably by this time
replaced by a gun of 8 inches in diameter), to propel a dart through
the air, in the front of which dart there is a metallic chamber
containing dynamite. Although no doubt the best engineer is the man
who does good work with bad materials, yet I presume we should not
recommend any member of our profession to select unsuitable materials
with the object of showing how skillfully he can employ them. On
the contrary, an engineer shows his ability by the choice of those
materials which are the very best for his purpose, having regard,
however, to the relative facilities of carriage, to the power of
supply in sufficiently large quantities, to the ease with which they
can be worked up or built in, and to the cost.

USES OF CEMENT.

Probably few materials have been found more generally useful to
the civil engineer, in works which are not of metal, than has been
Portland cement. It should be noticed that during the last twenty-two
years great improvements have been made in the grinding and in the
quality of the cement. These have been largely due to the labors
in England of o