marks of fire. There can he 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. Its from six to seven feet in thickness. In some places the fragments were cemented together with a soft decomposing 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 haying a mere projection to catch the ligatures, 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, There can be no doubt from the position of these remains, that man occupied the spot before the accumulation of the overlying débris. 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 considerable 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 small 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.
THE ABRADING AND TRANSPORTINGPOWER OF WATER
I.—MECHANICAL PROPERTIES OF WATER
IT is not my intention to lay down definite rules or formulæ regarding the Row 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," 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 Artizan 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: "The abrading and transporting power of water increases in some proportion as the velocity increases, but decreases as the depth increases."
The first question that arises iv 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 substances; for instance, in all bodies of a fatty nature the facility for wetting is very slight; and different temperatures 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 this 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 :o 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 matier, 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 he 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 runs down an inclined plane, the solid particles cannot by their 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 suspension in some proportion according to the load. On large rivers where this proportion may be only 11000 or 12000 part of the weight of water in motion, the retarding force may not he appreciable by the most careful experiments; so when calculating, the discharge may de left out
