Page:Lowell Hydraulic Experiments, 4th edition.djvu/38

 also planed, the slope of which, on an average, was $3⁄16$ inch in a height of $5⁄8$ inch; — the remainder of the casting was unplaned. The crest of the weir H was $3⁄8$ inch thick, and was horizontal. The upstream edge, at H, was a sharp corner. The ends of the weir K, figures 1, 2, and 3, were of wood, and of the same form as the crest H, except that there was no bevelled part corresponding to I, figure 4. The crest of the weir H was about 6.5 feet above the floor of the wheelpit. The ends of the weir K projected from the walls of the wheelpit, and also from the central pier, a mean distance of 1.235 feet. The length of the bay D, was 8.489 feet, and of the bay E, 8.491 feet, making the total length of the weir 16.98 feet.

45. The depth of the water on the weir was taken in the gauge box C, figures 1 and 2, plate V., by means of the hook gauge L, which is represented in detail by figures 9, 10, and 11, plate IV.

The hook gauge is the invention of Mr. Boyden, and is an instrument of inestimable value in hydraulic experiments. All other known methods of measuring the heights of the surface of still water, are seriously incommoded by the effects of capillary attraction; this instrument, on the contrary, owes its extraordinary precision to that phenomenon. At figure 10, plate IV., the point of the hook A, is represented as coinciding with the surface of the water. If the point of the hook should be a very little above the surface, the water in the immediate vicinity of the hook, would, by capillary attraction, be elevated with it, causing a distortion in the reflection of the light from the surface of the water. The most convenient method of observing with this instrument, according to the experience of the author, is, first, to lower the point of the hook, by means of the screw, to a little distance below the surface; — then to raise it again slowly by the same means, until the distortion of the reflection begins to show itself, — then to make a slight movement of the screw in the opposite direction, so as just to cause the distortion to disappear; the point will then be almost exactly at the level of the surface.

With no particular arrangements for directing light on the surface, differences in height of 0.001 feet are very distinct quantities; but by special arrangements for light and vision, differences of 0.0001 feet might be easily appreciated.

As this instrument cannot be efficiently used in a current, it was placed in the box C, in which the communication with the exterior was maintained by the hole M,