thor offers his views and speculations. The author does not appear to be thoroughly acquainted with either of the above mentioned sciences, and quotes many of his geological facts loosely, or without fully understanding them. His speculations accordingly are for the most part not well founded, although presented along with some right views, and in an attractive manner.

6. Lecture Notes on Physics. By ALFRED M. MAYER, Ph.D. 112 pp. 8vo. In the discharge of his duties as Professor of Physics in the Lehigh University at Bethlehem, in Pennsylvania, Prof. Mayer has prepared a clear and concise synopsis of the fundamental principles and notions of physics in accordance with modern views and the recent progress of science, which will be consulted with interest by those whose duties lead them in the same direction. These notes have been republished from the Journal of the Franklin Institute where they have appeared during the past year as a series of articles.

7. Strength of Iron parts of Steam Machinery. By J. D. VAN BUREN, Jr., C.E. 70 pp. 8vo. New York, 1869. (Van Nostrand.)-This is an analytical discussion of the formulæ employed by mechanical engineers in determining the rupturing or crippling pressure in the different parts of a machine. The formulæ are founded upon the principle that the different parts of a machine should be equally strong, and are developed in reference to the ultimate strength of the material in order to leave the choice of a factor of safety to the judgment of the designer. Engineers will find this a valuable contribution to the literature of their profession.

Publications of the Geological Survey of California.-The publications of the Geological Survey of California issued up to this date, or nearly ready, are as follows:

GEOLOGY, vol. i, with 525 pages of text, in royal 8vo form, 81 wood engravings and 1 steel plate; containing a Report of Progress and Synopsis of the Field Work, from 1860 to 1864. Price in cloth, $7.50.

PALEONTOLOGY, vol. 1, with 263 pages of text, and 32 steel and lithographic plates, comprising descriptions and figures of the Cretaceous and Triassic fossils, by W. M. Gabb, and of the Jurassic and Carboniferous, by F. B. Meek. Price in cloth, $7.50.

PALEONTOLOGY, vol. 2, with 299 pages of text, and 36 lithographic plates, containing the remainder of the Cretaceous and the whole of the Tertiary Palæ ontology, by W. M. Gabb. Price in cloth, $7.50.

ORNITHOLOGY. The birds of California and the adjacent States and Territories will be described and illustrated in a work now in preparation, which will embrace two volumes. The illustrations of the first of these, including the land birds, are now completed, and the volume is in press and will be ready for delivery before the end of the year. The illustrations are on wood and consist of a full-length figure of one species of each genus, with a diagram illustrative of the details of its external anatomy, and a full-sized figure of the head of each species carefully colored in the text. This mode of illustration is believed to be that best adapted to make the work one of high scientific and practical value, at the lowest possible cost. The work is based on the manuscript and collections of Dr. J. G. Cooper, Zoologist of the Survey, and edited by Prof. S. F. Baird.

MINING STATISTICS, No. 1, comprising a Tabular Statement of the Quartz Mines and Mills between the Merced and Stanislaus Rivers, in 1865, by A. Remond. Price 50 cents.

GEOGRAPHICAL CATALOGUE OF THE MOLLUSCA found west of the Rocky Moun tains, between latitudes 33° and 49° North, by J. G. Cooper, M. D., 40 pages (for check list and labels). Price 50 cents.

THE YOSEMITE GUIDE-BOOK. A work in one volume, 8vo, elegantly printed, and illustrated with wood-cuts (taken from Geology, volume 1), and two elabo rate topographical maps of the region described, namely the Yosemite Valley and its surroundings. Price in cloth, with maps in pockets, $5.00.

THE YOSEMITE BOOK. Nearly the same text as that of the Guide-Book, and with the same maps; but superbly printed in 4to form with 28 photographs by Watkins, taken expressly for this volume, of which the edition is strictly limited to 250 copies. Only a few copies still on hand. Price, in half morocco binding, gilt edges, $30.00.

MAP OF THE VICINITY OF THE BAY OF SAN FRANCISCO:-scale two miles to one inch, size four feet by three, in two sheets. Price, mounted on cloth, and varnished, and municipal township boundaries colored, $6.50: mounted on cloth, cut to fold, for traveller's use, in cloth case, $6.00. Printed on thin paper, not mounted, but folded in cloth case, $5.00.

MAP OF THE SIERRA NEVADA adjacent to the Yosemite valley:-scale two miles to one inch, size thirty inches by twenty. Price, on thin paper, in cloth case, $1.50.

The agents for the sale of the publications of the Geological Survey of California are for the Eastern States and Europe, Messrs. B. Westermann and Co., 471 Broadway, N. Y.; for the Pacific States, Messrs. H. Payot & Co., 640 Washington St., San Francisco.

Cambridge, Mass., June 1st, 1869.

Astronomical and Meteorological observations made at the United States Naval Observatory during the year 1866. Rear-Admiral Charles H. Davis, Superintendent. Published by authority of the Hon. Secretary of the Navy. 474 pp. 4to. Washington, 1868.

Paris Universal Exposition, 1867. Report upon Wool and Manufacturers of Wool, by E. R. Mudge, United States Commissioner, assisted by John L. Hayes, Secretary of the "National Association of Wool Manufacturers." Pp. 143, 8vo. Washington, 1868.

Sugli Spettri Prismatici dei Corpi Celesti. Memoire del R. P. A. SECCHI, Direttore dell' Osservatorio del Collegio Romano. pp. 70. Roma, 1868. A work containing many original observations.

PROCEEDINGS BOSTON SOC. NAT. HIST. Vol. xii.-p. 267, Tin Ore at Winslow, Me.; C. T. Jackson.-p. 268, Description of a Shark (Carcharias tigras Atwood); N. E. Atwood.-p. 269, Descriptions of North American Bees, No. 2, Andrenida; E. T. Cresson.-p. 274, Notes on new or little known species of American Cancroid Crustacea; S. I. Smith.-p. 289, Notes on the Concentric Structure of Granitic Rocks; N. S. Shaler.—p. 294, Synopsis of the Birds hitherto described from the Hawaiian Is.; S. B. Dole.-p. 310, Descriptions of some Extinct Fishes previously unknown; E. D. Cope.-p. 321, Notice of the Hemiptera obtained by the Expedition of Prof. James Orton in Ecuador and Brazil; P. R. Uhler.—p. 327, List of the Coleoptera collected by Prof. Orton; G. D. Smith.-p. 330. Notes on the Orthoptera collected by Prof. Orton on either side of the Andes of Equatorial South America; S. H. Scudder.-p. 345, A study of the Gigantic Lobe-crested Grasshoppers of South and Central America; S. H. Scudder.-p. 357, Use of the Microscopes; H. Hagen.-p. 361, Double Plate of Aulacodiseus oreganus, and Diatoms from Ashley River, S. C.; R. C. Greenleaf.-p. 364, Traces of ancient operations in the Oil-region of Penn; W. H. Niles.-p. 366, Notes on Mexican Pompilida; E. T. Cresson.-p. 380, Limits of Genera; C. S. Minot.p. 381, On new and imperfectly known Echinoderms and Corals; A. E. Verrill. -398, Antiquity of Man as shown by excavations at High Rock-Spring, Saratoga, N. Y.; H. McGuier.-p. 400, The Rough-winged Swallow and the Yellowbellied Flycatcher; P. R. Hoy.-p. 401, Method of accommodation in the eyes of Birds: B. J. Jeffries.—p. 402, Halibut and Blue-fish; N. E. Atwood.-p. 404, Report upon a Collection of Diurnal Lepidoptera made in Alaska; S. H. Scudder. -p. 408, Notice of a new cave insect (Hadencecus); S. H. Scudder.-p. 409,* Boreus brumalis Fitch; F. G. Sanborn.-Index to the vol.

THE

AMERICAN

JOURNAL OF SCIENCE AND ARTS.

[SECOND SERIES.]

ART. XV.-On the nature and duration of the discharge of a Leyden jar connected with an induction coil; by OGDEN N. ROOD, Professor of Physics in Columbia College.

[Read before the National Academy of Sciences, August, 1867.]

PART 1ST.

WHEN the terminal wires of an induction coil of sufficient size are connected with the exterior and inner coating of a Leyden jar, it is well known that the act of breaking the primary current charges the jar with electricity, and that a discharge will take place if the distance between the electrodes is not too great, otherwise the union of the two electricities is effected backward in the coil itself. The brightness of the light, and the ease with which it is generated, render it the most convenient means in our possession for the production of a nearly instantaneous illumination, while its richness in the violet and ultra-violet rays, make it suitable for experiments on fluorescence and phosphorescence. Before, however, it can become useful for these and other delicate investigations, involving the appreciation of minute intervals of time, it is of course necessary that the nature and duration of the discharge itself should be studied, a matter which, so far as I know, has thus far not received attention.

The case of a jar charged by an ordinary frictional machine is quite different from the above: here, during the the act of charging and discharging, the two metallic coatings of the jar are entirely insulated, while when the induction coil is used in AM. JOUR. SCI.-SECOND SERIES, VOL. XLVIII, No. 143.-SEPT., 1869.

the manner above indicated, these coatings are during both acts in metallic connection, a circumstance which alone might be supposed to modify the duration of the discharge and its nature; besides this, it is known that the electricity from the coil continues to flow for a considerable fraction of a second along the terminal wires into the jar, which fact renders it questionable whether the duration of the discharge of the jar itself may not be proportionately lengthened. Hence it is evident that it would not be safe to conclude without experiment, that the results obtained by Feddersen with jars charged by ordinary frictional electricity, were applicable to those connected with an induction coil.

Historical.-In the year 1835, Wheatstone published in the Philosophical Transactions, part II, page 583, an account of his celebrated experiment on the duration of the discharge of a Leyden jar charged by a common frictional machine. The light from the spark was received directly on a plane mirror revolving at rates between 30 and 800 times in a second; the mirror was driven by a set of multiplying wheels connected by strings. This apparatus was constructed by Mr. Saxton of Washington, who at that time was residing in London. The eye of the observer was placed near the mirror, and as the image of the spark was not sensibly drawn out by the rotation of the mirror, Wheatstone concluded that its duration was less than the one millionth of a second, a result which was accepted by the scientific world for about a quarter of a century, passing unquestioned till the publication in 1858 of the first of an admirable series of investigations of this subject by Feddersen; (Pogg. Annalen, Bd. 103, seite 69.) This physicist employed a concave mirror with a radius of half a meter, driven by a train of toothed wheels, and obtained as high a rate of rotation as one hundred per second; the spark was generated in one of the conjugate foci of the mirror, and its image, formed at the other conjugate focus on a plate of ground glass, was in a condition to be conveniently seen and measured by the naked eye or to be photographed. It was found that the image of the spark was drawn out by the revolving mirror into a whitish streak, from 20 to 30 millimeters long, which when a large amount of electricity was employed, was still farther lengthened by the addition of a red tail of about the same dimensions, this latter being due to the gradual cooling of the heated particles. With a jar of 2.2 feet inner coating, the distance of the copper balls which served as electrodes being 1.5 millimeters, and all the connections as short as possible, the duration of the discharge exclusive of the red tail was '00004 of a second. It was also

found by him, that an increase in the size of the coating of the jar or of the "striking distance," prolonged the duration of the discharge; so, for example with a spark 3.75 millimeters long, the duration of the whitish portion was 00007 of a second.

A series of observations detailed at the end of this article, will point out a probable explanation of the great discrepancy existing between the results of Feddersen, which are undoubtedly correct, and those of Wheatstone. Finally, Feddersen in the course of his investigation found abundant proof of the oscillatory nature of the electric discharge, which had been predicted from the results of a set of experiments by Prof. Joseph Henry as far back as 1842.

Apparatus.-I arranged in the first place an apparatus like that of Feddersen, but it was soon found that to attain a rotation of one hundred per second, it was necessary in my case to employ a weight of 200 pounds, and in addition when using this form, I labored under the disadvantage of not being able to vary the size or curvature of the mirror readily; in short it was found to lack flexibility, a quality more necessary in the present case than in the investigation which engaged the attention of the German physicist. On this account I decided to employ a plane revolving mirror with a stationary achromatic lens. The addition of an extra wheel, with a few other mechanical changes, converted the train of wheels furnished by Duboscq for Becquerel's phosphoroscope, into an admirable means for driving the revolving mirror, and with a weight of 30 pounds, 300 rotations per second were readily obtained, the motion being smooth and sufficiently uniform. In order to measure the rate of rotation, the cylinder on the lowest wheel was made to wind up a fillet of paper, upon which dots were made by an electro-magnetic apparatus regulated by a seconds pendulum, when a simple calculation furnished the rate of the wheel to which the mirror was attached. By this method it became possible to put to a sharp test the regularity of the rate of the train, which was found in all cases to be considerably greater than was at all necessary. These determinations were afterwards made with a watch having large second spaces, the dots being made by hand, as the refinement of a seconds pendulum and battery was found for my purpose to be superfluous. The mirrors employed were always plane, being sometimes silvered at the back, while for the most delicate observations the rear surface was painted with black varnish, so as to destroy the second reflection. They varied in size from 1.5 inches square, down to 2 by 5 in., and were used sometimes double on the axis back to back, and at other times four mirrors were em