EXTRAOEDINARY SHIPS. 
9 
and without any outer plate ; consequently the lower or sub- 
merged half of the hull is naturally hea-vier than the upper. 
In addition to which^ we know that a loaded bowling-ball 
will not roll_, but will oscillate backwards and forwards until it 
comes to rest. The engines and cargo of the cigar-ship 
practically load her_, and no probable amount of lateral motion 
from the sea is at all likely to turn her over. 
The primary consideration of the applicability of a screw- 
propeller to the work it has to do is its pitch ; that is the 
length in which the screw- whorl makes a complete turn round 
its central axis : and this pitch ” is either finer or 
coarser/'’ as the length of the axis 
[a h) is shorter or longer; or^ in other 
words, as the turns of the screw are 
closer together or further apart. 
It is evident that if there be no rig. is. -pitch of screw. 
slip — that is, if the screw retains 
full hold upon the water, as a carpenter^s screw does 
upon a board — that in every revolution of the screw- 
propeller a distance of the length of the axis of the full turn 
of the screw will have been accomplished by the vessel. If 
thus the pitch of the screw be 40 feet — that is, if from 
u to 6 be 40 feet — then every full revolution of the screw 
will — if there be no slip — advance the vessel 40 feet 
through the water. 
But the dense resistance of the water to the passage of the 
vessel forces the flat surface of the screw-propeller against the 
water in which it revolves, and the water, being displaceable, 
gives way proportionately to the pressure upon it ; and conse- 
quently, there is a certain amount of what is expressively and 
correctly termed slip,^^ in many cases amounting to as much 
as thirty-five per cent. Two considerations are thus involved ; 
whether there is less loss by slip in a screw of coarse pitch 
working at a slower rate, or in one of fine pitch working with 
quicker revolutions. The effects of fine and coarse pitches 
are being practically tried by Mr. Winan, and until the experi- 
ments are completed, it would be premature to speculate upon 
what the half-immersed propellers of the cigar- ship are 
likely to accomplish. 
We now turn to another class of vessels altogether, — one 
that has much more of a commercial than of a scientific cha- 
racter — Connector- ships.-’* The present model vessel which 
one often sees on the Thames, and occasionally at some of our 
seaports, may be most fitly termed a sea-train. She consists 
of an after or engine compartment, and a fore or bow com- 
partment. Between these two ends a greater or less number 
of ordinary carrying compartments — convex in front, concave 
