Depth below surface. Depth below sur face. Depth below surface. MAXIMUM. TABLE XXXVI.-Mean pulsations St. Clair River, August 21, 1868, 208 feet from base; depth of river, 28 feet; wind down, 11.9 miles per hour. August 22, 1868, 1,411 feet from base; depth of river, 42 feet; wind up, 0.7 miles per hour. Duration in No. of MAXIMUM. Revolutions per second. September 14, 1868, 487 feet from base; depth of river, 45 feet; wind up, 5.0 miles per hour. Velocity of river, feet per second. The above table, however, shows the duration and mean number of revolutions near the maximum and minimum. The time occupied by a pulsation varies from a few seconds to over a minute, not appearing to be at all regular. They are greater at the bottom than at the surface, and therefore cannot be accounted for on M. Bazin's theory, hereafter noticed, "as due to the tumultuous action at the surface caused by friction on the air." I found that eddies made no difference with them, as in the canal at Odgensburg they were about the same in the stiller portions as at the grating, where the water forced through from the dam. They were also perceptible in the tail-race of a mill, where the water was quite shallow, and rushing with great velocity over a strong bed, and in the St. Lawrence where the volume of water is so great and the current slow. Savart experimented on the form of discharge of water flowing from Revolutions per second. Velocity of river, feet per second. an orifice in the bottom of a reservoir, and found that though the vein was of a cylindrical form between the orifice and the contracted section, yet at a small distance beyond the latter point a succession of shrinkings and swellings of the vein took place, which further became drops alternately large and small, there being between each drop a space eight or ten times its mean diameter. The form of the drops oscillated between a prolate and oblate spheroid. Thus we see that water, when flowing freely from an orifice, under a constant head, is subject to continual variations apparently caused by the action of the air, and it may be that the motion of the water in tubes or open channels is affected in the same manner, and thus cause these pulsations. There is another cause which would perhaps account for the large pulsations, and that is the continual oscillation of the water in the lakes. In calm summer days, without any apparent cause, the water is continually rising and falling. This can be seen on the sheets of the seltregistering water gauge, or in small streams at the head of deep bays. In the latter, if the fall is not large near the lake, the current is continually changing up and down; the time of one ebb and flow being from ten to twenty minutes. These changes of level have also been noticed in the lakes of Switzerland, and in the Caspian Sea, so that they seem to be incident to all large bodies of water. They have been attributed to the varying atmospheric pressure, but they are more rapid than any barometic oscillation, and they are probably more due to disturbances of the crust of the earth. Humbodlt was of the opinion that earthquakes were continually occurring in all parts of the world, and that it was only the larger ones which were noticed; and astronomical observers now find that their instruments are subject to perturbations which are only to be accounted for on some such theory. Bazin again says, (Hydrauliques, pages 23 and 25,) "These irregular movements appear to be preferably produced in the greatest sections, but it is, above all, in the neighborhood of the surface that these variations, or, better called, perhaps, the disturbances of the velocities, are the most remarkable." And he further says, "That the changes in the curves of equal velocities, seen in a rectangular tube, and an open canal of the same width and half the height heretofore mentioned, cannot be accounted for by the resistance of the air, but must be due to the interior movements of the fluid. For, in a tube, the symmetry and invariability of the section establishes among the molecules a kind of solidity, which contributes to make the velocity regular. In an open canal, on the contrary, the absence of pressure in the surface of the current, the want of symmetry in the section, which freely permits the augmenting or diminishing of the incessant, from which the current is not exempt, favor, without doubt, the productions of these tumultuous movements in the superior fillets." Whether, as he seems to intimate, there are no pulsations in a closed tube, I am not able at present to say, as I have had no means of testing it, but, as will be seen from the tables, there is no increase of the pulsations near the surface, but generally they are the greatest when we approach the bottom and sides, and they seem to be about the same in the generally even-flowing river as in the small, eddying canal. Dubuat, in the course of his experiments on the action of water on different substances placed in his experimental canal, covered the bottom of it with a layer of sand, which moved with the water under a velocity of one foot a second. After some time the sand presented a series of beautiful undulations or ridges 0.4 foot broad. The grains of sand, forced on by the current, rose to the top of the ridges, and, after falling down by their own weight to the base of the next ridge, were again lifted to the summit; he found that half an hour was required for a particle to traverse a complete ridge, so that the rate of motion would be about twenty feet in twenty-four hours. These ridges seem to be the effect of the pulsations, for the formation of the sand dunes by the wind is probably due to the fact that its force is irregular, the sand borne forward during its greater velocity being dropped when that velocity decreases; an irregularity in the surface being thus produced, which will cause greater resistance to the passage of the next sand-bearing current, and thus cause the deposition of more particles upon its surface. But more data is required before we can reason intelligently upon this subject. In the following table is given the rain-fall on the water-shed of the lakes up to December, 1868. The table is a continuation of that given last year: TABLE XXXVII.—Rain-fall in the region of the great lakes. TABLE XXXVIII.—Bi-five day means of the height of the water for the several places of observation, 1868. The following table gives the stage of water for the season at the several stations, below the high water of 1838: Niagara. OF 1838. St. Law- Sault Ste Marie. In the meteorological report is given, the evaporation at all the known points. The past year I made as extended observations as possible of the difference between the evaporation in the river and that on the land, at the meteorological observatories. These observations were somewhat difficult to take, as the river evaporator had to be carefully watched to keep it from slopping. As the water in it was constantly in motion the evaporation was probably somewhat less than if it had been still, and, as the evaporation at the river stations was larger than that at the other stations on the lakes, the ratio between the evaporators given in the table is taken as correct for the whole lakes, though, on account of the temperature being greater in the river than on the lake surface generally, it should otherwise be somewhat reduced. WATER BELOW THE HIGH WATER TABLE XXXIX.-Showing the difference between the simultaneous readings of an evaporator. at the meteorological station and one placed in the river St. Clair, 1868. TABLE XL.-Showing the difference between the simultaneous readings of an evaporator at the meteorological station and one placed in the river Niagara, 1868. |