acts very advantageously in aid of
sails, for not only does the operation of the sails in reducing the
resistance of the hull virtually increase the screw's diameter, but the
screw, by reducing the resistance which has to be overcome by the sails and
by increasing the speed of the vessel, enables the sails to act with
greater efficiency, as the wind will not rebound from them with as great a
velocity as it would otherwise do, and a larger proportion of the power of
the wind will also be used up. In the case of beam winds, moreover, the
action of the screw, by the larger advance it gives to the vessel will
enable the sails to intercept a larger column of wind in a given time. It
appears, therefore, that the sails add to the efficiency of the screw, and
that the screw also adds to the efficiency of the sails.

611. _Q._--What is the comparative cost of transporting merchandise in
paddle steamers of full power, in screw steamers of auxiliary power, and in
sailing ships?

_A._--That will depend very much upon the locality where the comparison is
made. In the case of vessels performing distant ocean voyages, in which
they may reckon upon the aid of uniform and constant winds, such as the
trade winds or the monsoon, sailing ships of large size will be able to
carry more cheaply than any other species of vessel. But where the winds
are irregular and there is not much sea room, or for such circumstances as
exist in the Channel or Mediterranean trades, screw vessels with auxiliary
power will constitute the cheapest instrument of conveyance.

612. _Q._--Are there any facts recorded illustrative of the accuracy of
this conclusion?

_A._--A full paddle vessel of 1000 tons burden and 350 horses power, will
carry about 400 tons of cargo, besides coal for a voyage of 500 miles, and
the expense of such a voyage, including wear and tear, depreciation, &c.,
will be about 190_l_. The duration of the voyage will be about 45-1/2
hours. A screw vessel of 400 tons burden and 100 horses power, will carry
the same amount of cargo, besides her coals, on the same voyage, and the
expense of the voyage, including wear and tear, depreciation, &c., will be
not much more than 60_l_. An auxiliary screw vessel, therefore, can carry
merchandise at one third of the cost of a full-powered paddle vessel. By
similar comparisons made between the expense of conveying merchandise in
auxiliary screw steamers and sailing ships on coasting voyages, it appears
t