The water escapes in a thin and even sheet as soon as the tractive stress exceeds 2,000 kilogrammes; and the intensity and size of the eddies from the boat sensibly diminish in measure as the speed increases.
The interesting experiments made by Mr. Pictet seem, then to clearly establish the fact that the forms deduced by calculation are favorable to high speeds, and will permit of realizing, in the future, important saving in the power expended, and, consequently, in the fuel (much less of which will need to be carried), in order to perform a given passage within a given length of time. Thus is explained the great interest that attaches to Mr. Pictet's labors, and the desire that we have to soon be able to make known the results obtained with such great speeds, not when the boat is towed, but when its propulsion is effected through its own helix actuated by its own engine, which, up to the present, unfortunately, has through its defects been powerless to furnish the necessary amount of power for the purpose.--La Nature.
INITIAL STABILITY INDICATOR FOR SHIPS.
For a vessel with a given displacement, the metacenter and center of gravity being known, it is easy to lay off in the form of a diagram its stability or power of righting for any given angle of heel. Such a diagram is shown in Fig. 3, in which the abscissæ are the angles of the heel, and the ordinates the various lengths of the levers, at the end of which the whole weight of the vessel is acting to right itself. The curve may be constructed in the following manner: Having found by calculation the position of the transverse metacenter, M, for a given displacement--Figs. 1 and 2--the metacentric height, G M, is then determined either by calculations, or more correctly by experiment, by varying the position of weights of known magnitude, or by the stability indicator itself. Suppose, now, the vessel to be listed over to various angles of heel--say 20 deg., 40 deg., 60 deg., and 80 deg.--the water lines will then be A C, D E, F K, and H J respectively, and the centers of buoyancy, which must be found by calculation, will be B1, B2, B3, and B4. If lines are drawn from these points at right angles to the water levels at the respective heels, the righting power of the vessel in each position is found by taking the perpendicular distances between these lines and the center of gravity, G. This method of construction is shown to an enlarged scale in Fig. 2, where G is the center of gravity, B1 Z1, B2 Z2, B3 Z3, and B4 Z4 the lines from centers of buoyancy to water levels; and G N, G O, and G P the distances showing the righting power at the angles of 20 deg., 40 deg., and 60 deg. respectively, and which to any convenient scale are set off as the ordinates in the stability curve shown in Fig 3.
STABILITY INDICATOR FOR SHIPS. Fig. 1.
Having obtained the curve, A, in this manner for a given metacentric height, we will suppose that on the next voyage, with the same displacement, it is found that, owing to some difference in stowage, the center of gravity is 6 in. higher than before. The ordinates of the curve will then be G¹ N¹ and G¹ O¹--Fig.2--and the stability curve will be as at C--Fig. 3--showing that at about 47 deg. all righting power ceases. Similarly, if the center of gravity is lowered 6 in. on the same displacement, the curve, B, will be found, and in this manner comparative diagrams can be constructed giving at a glance the stability of a vessel for any given draught of water and metacentric height.
