Further, that with the pressure on and normal to the blades tending
to bend them towards the bow, such a displacement in that direction
might throw the screw out of pitch. In some cases, perhaps, the real
slip may differ from the apparent by a larger amount than it other-
wise would be from such causes, especially when the blades are of
bronze and thin.
All these possible causes were, however, examined and discounted
years ago, and the consensus of opinion settled the matter in the way
above indicated. If, however, wake currents caused by the ship were
the cause — and no one will doubt that they are most disturbing
elements in their effects on screws — how did it come about that in the
same ship two screws having the same diameter should in one case
run with positive slip and in the other with negative ? Further, what
was there to make wake currents so very pernicious from 1864 to
1870 ? for although negative apparent slip had been observed as early
as 1841 to a small extent in H.M.S. " Rattler," and in 1853 on H.M.S.
" Miranda" when on her trial trip, it was 7'85 per cent, slip; it was
not, however, of common occurrence till 1864, when H.M.S. " Achilles,"
a long and fine-lined ship, showed as much as 13*0 per cent, negative



SLIP ; CAVITATION AND RACING. 6 1

slip in Stokes Bay ; a similar but somewhat longer ship, H.M.S.
