328 
POPULAR SCIENCE REVIEW. 
Two very important papers on apparent negative slip of screw propellers 
were read, one by Professor W. J. M. Rankun, and the other by Mr. W. 
Froude. In the former, the existence of negative slip is explained by the 
fact, that the position of the propeller places it in the crest of a following or 
filling wave under the ship’s counter, so that the water which the screw lays 
hold of has a temporary forward velocity over and above the permanent 
velocity of the wake ; that temporary forward velocity may be many times 
greater than the permanent velocity of that current whose momentum is 
equivalent to the resistance of the ship ; and thus any amount of negative 
slip may be accounted for. In Mr. Froude’s paper the negative slip is attri- 
buted to the action of the screw on the dead-water carried by the ship in 
front of the stern-post, and which water has to be suddenly removed twice 
at least in each revolution of the screw. The block of dead-water thus 
carried, in the screw-well of a timber line-of-battle ship, is estimated at 
from 5 to 6 tons. Its dispersion, as the blade of the propeller sweeps past 
the stern-post, is accompanied with a violent shock on the screw-shaft, 
whilst the dispersed dead-water is instantaneously replaced by a fresh 
volume, to which the velocity of the ship must be rapidly imparted ; and 
the communication of velocity to the water of replacement forms an adven- 
titious addition to the ship’s resistance. In this struggle between the 
propeller and the dead-water, although a precise balance is maintained 
between the propulsive shock delivered and the adventitious resistance 
called into play, yet a great deal of force is exerted to no purpose by the 
propeller and the mean speed of rotation of the propeller must undergo 
great reduction, in such a manner as to be capable of exhibiting, in almost 
any degree, the phenomenon of negative slip. In other words, whilst in 
reference to the speed of the propeller whilst acting in the free-water 
outside the dead-wood, the slip is always positive ; in reference to the mean 
speed, including the retardations in passing the dead-wood, the slip is 
negative. Mr. Froude suggests as a means of testing his theory, the chrono- 
metric registration of the speed of rotation of the screw shaft, and as a 
means of obviating the sources of loss which his theory indicates, he would 
place the propeller not merely abaft the rudder but at some tangible distance 
clear of it. In some small scale experiments Mr. Froude obtained a result 
from thus removing the propeller equivalent to doubling the horse power. 
Captain L. G. Heath R.N., who has had great opportunities of observing 
the effect of shot on targets, proposed a new system of armour-plating, in 
which the more or less perishable wood-backing is entirely dispensed with. 
He places the armour-plate, made as heavy as possible, on horizontal girders, 
so as to carry it about 14 inches in front of the ordinary skin of the ship, 
leaving an air space of that depth between. He anticipates that shot will 
be broken up in passing through the armour-plate, and will merely splash 
the vital part. 
During the meetings drawings of a self-registering floating apparatus, for 
measuring the height of waves, was exhibited ; the instrument having been 
contrived and used with success by Admiral Paris, of the French Navy. 
Channel Ferry . — We mentioned in our last number, the proposed tunnels 
between this country and France. Messrs. A. and I. Inglis have now 
published, as a more feasible scheme, the details of a ferry steamboat of 
