With both of in the boat, without raising the meter out of the water. these instruments the discharge of the stream was measured at one of the cross section lines. The measurements were made in various ways. Sometimes the average velocity in a vertical was determined by using the instrument uniformly from top to bottom, and back several times. In other cases the velocity was determined at certain points in the cross section, at different depths and at different distances from the banks. All these measurements were plotted and the discharge of the stream deduced from them. The results of the work were very satisfactory, and the students obtained a good knowledge of the methods employed. The actual quantities deduced were not, however, of much practical value, and could not be easily compared with one another, for the reason that during the time of the measurements the stream was unfortunately rapidly changing its level in consequence of severe and sudden rains which had occurred over certain parts of its basin. During the first part of the measurements the river was rising at a rapid rate, while during the last part it was falling equally rapidly. It is unnecessary to say that under such circumstances any measurement of flow is more likely to be in error, and is of less value than if taken while the river is at a uniform state. The object in this case, however, was not to find how much water was flowing in the river, but to teach the students how to measure the flow of the stream, and this was accomplished satisfactorily. MEETING 381. Peculiar Rotary Motions found in Lightning and other Electrical Currents. BY MOSES GREELY PARKER, M. D. The 381st meeting of the SOCIETY OF ARTS was held at the Institute on Thursday, December 18th, at 8 P. M., Hon. J. A. Dresser in the chair. After the reading of the records of the previous meeting, the chairman introduced Dr. Moses G. Parker, of Lowell, who read a paper on "Peculiar Rotary Motions found in Lightning and other Electrical Currents." Dr. PARKER said: Photographing lightning and making good pictures, representing the forked and irregular light as it shoots. across the sky, or from cloud to earth, has become quite common, and is easily done on a dark night; having first focussed the lens for distant objects, one has only to point it in the direction he expects the flash to appear, expose the plate, and wait for the lightning. If the night is dark, one can obtain several flashes of lightning on the same plate, as they often occur near together. The only difficulty is to have the flash come within the field of the lens. Photographing the current itself so as to get detail in the track, showing how the electrical currents travel, is quite another thing. This we have been able to do, and present some results, as seen in figures 1, 2, 3, and 4. From these we see that the electrical current may travel without dividing, or it may divide and sub-divide, twist and meander in its passage from cloud to earth, its image on the negative presenting such a variety in form that many names have been given descriptive of its general appearance, without any reference to the real motion of the current itself. Three of these motions I have observed, viz., the twisted, the curled, and the straightforward. The twisted motion, as seen in figures 1, 3, and 4, resembles a loosely-twisted rope. It twists both ways, usually from left to right, Fig. 1. Reproduced from photograph, showing the attraction and repulsion of the currents, also the rope-like twist. as the twining vine winds around its support. There are exceptions to this rule in the electrical as well as the vegetable kingdom, for we find it twisting not only both ways (i. e., from left to right, and right to left), but it reverses its motion in the same course. The curled motion seen in figure 2 resembles a twisted ribbon or shaving as it curls from the carpenter's plane, and in some respects is most remarkable. Fig. 2. Reproduced from photograph. Ribbon Lightning, showing its curled motion. The straight presents straight lines in its track, and is evidently traveling with great force. [The Doctor here made quotations from the report of the Committee of the Royal Meteorological Society of London, issued last year, which classifies lightning under six different names, viz., stream, sinuous, ramified, meandering, beaded, and ribbon lightning.] Some two years ago, while experimenting with electricity, I obtained a photograph showing the dividing and twisting rotary motion of the electrical current, as seen in figure 1. My plate was not quite in focus, but the image is sufficiently sharp to show that the current divides and rotates, not only on itself, but upon its fellow. Knowing, as we do now, that the current has a rotary motion, we can see, in the main track, indications of this motion, that would be impossible for any jarring of the camera to produce. Further investigation disclosed the three motions before mentioned, to illustrate which I have, by permission of A. H. Binden, taken figures 3 and 4 from his remarkable photographs of many flashes of lightning, about which, it was truly said in the Boston Herald of July 29, 1888, “Mr. Binden has been singularly fortunate in securing, with his two plates, photographic reproduction of all the typical forms of lightning flashes mentioned in the committee's report." The lightning flash, examined as a whole, is seen to leave the cloud and reach the earth in an irregular, twisting, rotary manner, throwing off branches as it goes; these also twisting, rotating, Beaded, Fig. 3. Reproduced from photograph, showing rotary mot on. and sub-dividing into the sinuous, ramified, meandering, beaded or chapleated, and ribbon lightning, mentioned in the Royal Society's report, while the main current, rotating as it goes, finally enters the earth in a divided form. as seen in figures 3 and 4, which plainly shows this twisting rotary motion in the main current as well as in its branches. Stream lightning is well described by its name alone. In this form I find what I have called the straightfor its photo ward motion, graphs show almost straight lines without the curves indicating the rotary motion. Sinuous, ramified. and meandering lightning, figures 3 and 4. are all very much alike, if we grant that which we can hardly doubt, viz., that they all may divide and sub-divide as they advance. In all of these we find a rotary motion, with a direction either from left to right, or right to left; in some branches both motions are found, and when well defined, resemble the twist of a rope. Beaded lightning, seen in the end of figure 3. has that about it that is much more interesting from a speculative point than either of Meandering, Sinuous and Ramified Lightning. the others. The explanation given in the Royal Meteorological Society's report of the beaded form barely explains all that we find in and on both sides of this bead, for we see the rotary motion of this current before entering the bead to be in one direction, and immediately after leaving the bead to be in the opposite direction, plainly indicating that the motion sometimes changes in the bead. Ribbon lightning, as seen in figure 2, has what I have designated the curled motion. In this one sees the resemblance to a curled ribbon. This current is evidently flat, with a motion that forms this ribbon into a curl, as seen at the end. It somewhat resembles the beaded form, inasmuch as it is seen to change its direction, thus Fig. 4. Photographic view of ramified and meandering lightning, with loops and rotary motion. |