156 Captain Hall’s Account of the Dundee Ferry . 
of it (to say nothing of what results from actual practice), very 
often stands more in the way of correct views, than it aids a just 
apprehension of the subject. It is practically important that this 
risk should be duly understood, and that seamen themselves should 
be on> their guard in reasoning on the subject. One instance may 
be mentioned of the errors into which naval men are liable 
to be led by constant familiarity with principles inapplicable to 
the subject under discussion. In ships moved by sails it is es- 
sential that there should be a certain draught of water, in order 
that she may offer an infinitely greater lateral resistance than she 
does in the direction of her length. This necessity arises out of 
the well known law, by which the wind acts on the plane of the 
sails, by which the effect is at right angles to that plane, at what- 
ever angle the wind may strike it. In the steam-vessel, how- 
ever, the effect of the paddle being always in the direction of 
the length, the same draught of water, which in ships is indis- 
pensable to resist the lateral tendency caused by the sails, is not 
required in steam-vessels, which may be said to have compara- 
tively no lateral tendency at all, since the force of the wind on 
their hulls and chimney is immaterial when compared with that 
of the paddles. The fallacious conclusions drawn from this mis- 
taken analogy, have led to serious practical errors. 
The result of the investigations which I instituted on the 
subject, is, that the Twin-boat may be established on the ferry 
between Newhaven and Fife, with perfect safety, and great 
advantage, not only as a matter of public convenience, but as a 
source of profit to the funds of the ferry. 
Of course, a twin-boat, that has to contend with such a sea as 
frequently rolls into the Firth, or is often caused by the preva- 
lent high south-west winds, must not only be more strongly 
built than the boat which has merely to cross the Tay at Dun- 
dee, but must also be impelled by much more powerful engines. 
Practical men, however, well qualified to give an opinion on 
the subject, conceive that great additional strength may easily be 
given to a boat of this construction, without such addition of 
weight of materials as will render the draught of water incon- 
venient for a low-water pier. One or two of the devices which 
have been thought of may be stated. The timbers which form 
the inner sides of the two divisions of the boat, instead of being 
