The Sun.
London, England, 1868-08-27 00:00:00

PHOTOGRAPHS OF PLANTS.

Professor KARL KocH called the, attention of the mem-
bers of the department of Zoology and Botany to the
necessity for having photographs in order to carry out
the systematic comparison of plaats, for some plants when
dead gave only a bad idea of physiognomy. The pro-
fessor pointed out that in the Botanic Gardens at Berlin
there was a collection of photographs of plants, and the
possessors of gardens and greenhouses were invited to con-
tribute photographs of interesting trees and plants.

MECHANICAL SCIENCE.
The probable connection between the resistance of ships
and their mean depth of immersion was the subject of a
communication to this section by Professor RANKINE, C.E.
The paper pointed cut that it was mooted some time ago
that when a wave in water was raised by a floating solid
body which was propelled at a speed greater than the
natural speed of the wave, the ridge of the wave
assumed an oblique position, and the wave ad.
vanced obliquely, so that while it travelled at its
own natural speed in a direction perpendicular to
its ridge-line, it at the same time accompanied the motion
of the solid body at a greater speed. The angle of
obliquity of the advance of the wave was such that its
cosine was the ratio of the natural speed of the wave to
the speed of the solid body. It was at the same time
pointed out that under those circumstances there was an
additional breadth of wave raised in each second expressed
by the product of the speed of the solid body into the
sine of the obliquity ; that in raising that additional
breadth of wave per second energy was expended, and
thus that a rapidly-increasing additional term was intro.
duced into the resistance to the motion of the solid body
BO soon as its speed exceeded the natural speed of the
waves which it raised. In a paper read to the Royal
Society in May 1868, it was shewn that for all waves
there was a relation between the natural speed
and the virtual depth of uniform disturbance ; that
is to say, the depth to which a uniform disturbance
equal to the disturbance of the surface particles would
have to extend in order to make a total volume of disturb-
ance of the water equal to the actual volume of disturb-
ance. That relation was, that the speed of advance of the
waves was that due to a fall of half the virtual depth. In
another paper,read to the Institution of Naval Architectsin
1868, it was shewn that any ship was probably accompanied
by waves whose natural speed depended on the virtual
depth to which she disturbed the water, and that conse-
quently when the speed of the ship exceeded that natural
speed there was probably an additional term in the resist.
ance depending on such excess. The object of Professor
Rankine in his communication was to call the attention
of the British Association, and especially of the committee
on steamship performances, to the probable existence
of this hitherto neglected element in the resist-
ance of ships, and to suggest that suitable observa-
tions and calculations should be made in order to discover
its amount and its laws. In an appendix to his paper, the
professor gave the results of the three operations he had
made of the speed of advance of the obliquely-diverging
waves raised by ships. The waves in each case were those
which follow the stern of the vessels, and the vessels
were all paddle-steamers. Care was taken to
observe the positions of the wave ridges where
they were beyond the influence of the paddle. The virtual
depth corresponding to the speed of advance of those
waves was calculated in each case, and it was found to
agree very nearly with the mean depth of imm 'rsion. Few
as the observations were, they were st,ffi.ient to prove the
existence of waves whose speed of advance depended on
the depth to which the vessel disturl. el the water. The
connection between those waves and the resistance
remained a subject for future investigation.