April 21, 1870] 
marks of fire. There can be no doubt that this stratum 
marks the place where the dwellers in the cave, during 
Roman or immediately post-Roman times in Britain, 
kindled their fires and cooked their food. Underneath is 
a talus of limestone fragments detached from the cliff by 
atmospheric action, like the superficial accumulation. Itis 
from six to seven feet in thickness. In some places the 
fragments were cemented together with a soft decompos- 
ing stalagmite. It rested on a layer of grey clay, of a 
thickness which at present has not been ascertained. At 
the bottom of the talus, and close to the entrance that is 
now being made into the chamber, there were found two 
rude flint flakes, a remarkably large lower jaw of bear, the 
broken bones of the Celtic shorthorn (Bos longifrons), 
and of the red-deer. On the 4th April a most remarkable 
bone harpoon was dug out from the same horizon. It is 
between four and five inches in length, and is furnished 
with two barbs on each side, arranged opposite each 
other, composing the head of the implement. The 
base presents a form of attachment to the handle 
which, so far as my knowedge extends, is new to Britain. 
Instead of having a mere projection to catch the liga- 
tures, there is a well-cut barb on either side that points in 
a contrary direction to those on the head. Were the 
bases of a barbed arrowhead and of a harpoon joined 
together, the resultant form would be analogous to the one 
in question. Therecan be no doubt from the position of 
these remains, that man occupied the spot before the 
accumulation of the overlying déérzs. Ample use for his 
harpoon he would find in the mere, now drained and 
turned into green fields, which are almost overlooked by 
the cave. So far as the work has proceeded there is no 
trace of metal at this horizon in the section. 
The value of the evidence hitherto obtained lies in the 
fact that the Roman stratum is separated from the lower 
level, in which the flints, harpoon, and bear were found 
by the talus of angular stones. And this in a rough way 
enables a computation to be made of the date of the lapse 
of time between them, if we allow that for a consider- 
able time past, immediately outside the historical epoch, 
the disintegration of the cliff has been equal, in equal 
times. For since, in twelve hundred years, to put it 
at the lowest, only a thickness of twenty-four inches has 
been accumulated above the Roman remains, it would take 
three thousand six hundred years for a deposit of six feet 
to be formed, and thus the harpoon and flint stratum 
would be about four thousand years old. The accuracy of 
this calculation is indeed injured by the possibility that 
the winter cold was more intense, and the splitting action 
of the frost greater in Pre-, than in Post-Roman times. 
Nevertheless, the change from the Arctic severity of the 
post-glacial winter, to the climate which we now enjoy in 
Britain, has been so gradual, and has been spread over so 
long a period, that it may be assumed to have been very 
smali in so short a time as four or five thousand years. 
This account is merely an outline of the results obtained 
up to April the 4th. The cave promises to be a rich one, 
and will probably add very much to our knowledge of the 
Pre-historic dwellers in Yorkshire. 
W. BoypD DAWKINS 
THE ABRADING AND TRANSPORTING 
POWER OF WATER 
I.—MECHANICAL PROPERTIES OF WATER 
je is not my intention to lay down definite rules or 
formule regarding the flow of water, but rather, by 
drawing attention to generally-acknowledged facts, to 
throw out suggestions which may serve to lead to the 
discovery of some general laws of practical use to the 
hydraulic engineer. 
In 1857 a paper was read by me- before the Royal 
Society of Edinburgh, “ On the Delta of the Irrawaddy,” 
NATURE 
629 
in which I expressed an opinion that depth somehow 
affected the abrading and transporting power of water. 
My experience of Indian rivers and canals during the 
succeeding ten years went to confirm this opinion, and 
before the Institution of Civil Engineers, as well as on 
two occasions before the British Association in 1868 and 
1869, I ventured to give expression to my views of this 
law, as affecting artificial rivers for irrigation, and the 
bridging of rivers which flow through the alluvial plains of 
Northern India. 
In the Artizax there have appeared during the last 
six months several short articles bearing on the same 
subject, showing how all questions relating to flowing 
water are affected by this supposed law, which may be 
stated as follows: “che abrading and transporting power 
of water increases in some proportion as the velocity in- 
creases, but decreases as the depth increases.” 
The first question that arises in this inquiry is—What 
is water in a mechanical point of view ? 
This may be briefly answered by saying that it is a fluid, 
the particles of which, though easily separated, do again 
unite, and exert a certain affinity towards each other, and 
also to other bodies, so that a certain amount of power is 
necessary to effect a separation. The attraction of the 
particles of water to other bodies varies with different sub- 
stances ; for instance, in all bodies of a fatty nature the 
facility for wetting is very slight ; and different tempera- 
tures also affect this property of water. This attraction or 
force is technically known as “ skin friction,” and deserves 
the most careful investigation ; for it is owing chiefly, if not 
altogether, to the fact that water has the power of abrasion, 
and it is this property which introduces the most difficult 
problems that a naval architect has to solve. 
The affinity of one set of particles of water to another 
set, may possibly be measured by noting the size of a 
drop of water which falls from a wetted surface of a 
given area. By thus determining accurately the weight 
of water a given area can support, some approximate 
results of an instructive character may be arrived at; but 
what adds to the complication of the question is, that the 
cohesion of the particles probably differs according to 
the temperature and the purity of the water experimented 
on. Thus, when water reaches the boiling point the 
affinity, it is believed, becomes very much lessened ; and, 
again, it is thought that with pure or distilled water the 
particles probably have less affinity to each other than with 
water less pure. This impurity may arise from various 
causes ; sewage, for example, would probably give much 
heavier drops from the same wetted area than rain water, 
in the same manner that drops of treacle are much larger 
than those of water; that is to say, the affinity, attraction, 
or cohesion of the particles is as a general rule increased 
by the introduction of foreign matter held in solution. 
With solid matter held in suspension a similar result is 
obtained, not by increasing the cohesion of the particles 
of water, but by increasing the surface area wetted ; for 
each grain of foreign matter, be its shape what it may, 
must have all its surface in contact with the water, 
This probably explains how a drop of mud should be so 
much larger than one of water, and, at the same time, it 
may possibly explain why thick muddy water, or more 
properly speaking, liquid mud, with the same section and 
slope, cannot travel so fast as water. 
From this it may reasonably be supposed, that when 
muddy water rung§down an inclined plane, the solid par- 
ticles cannot by tir own gravity sink so rapidly towards 
the bottom as to overcome the power dragging them 
in a different direction. As a consequence, the flow of 
water is retarded by having solid matter held in suspen- 
sion in some proportion according to the load. On 
large rivers where this proportion may be only yggq or 
sono part of the weight of water in motion, the retarding 
force may not be appreciable by the most careful experi- 
ments; so when calculating, the discharge may be left out 
