Imágenes de páginas
PDF
EPUB

7. Post-Pleiocene Fossils of South Carolina ; by Francis S. HOLMES, A.M. Nos. 1 to 5, in 4to, with two lithographic plates to each number. Price $2.00 per pumber.—This new work by Mr. Holmes, is published in the same style as the Pleiocene Fossils, and with similar beautiful illustrations. Figures of about 100 species are given in the 5 parts issued. The volume will be completed in 15 parts, 5 parts appearing annually. From Mr. Holmes we learn that the Post-pleiocene beds extend along the entire seaboard of the state of South Carolina, outcropping about 10 miles from the coast. At the Artesian well, in Charleston, the auger penetrated the stratum of blue mud containing Post-pleiocene shells at the depth of 21 feet. There was first 2 feet of mud ; then 2 feet or more of laminated blue sands and clays; then blue mud again, and so on for 49 feet 5 inches from the surface, when it reached the Eocene Marl. At Ashley Ferry, 10 miles northwest of Charleston, the stratum is only 1 foot thick, and rests upon Eocene Marl. The best locality is at Simmons's on Wadmalaw Sound, 10 miles from the sea-coast, as described in Tuomey's Geology of South Carolina, p. 189.

8. The Medical Application of Electricity; by William F. CAANNING, M.D. Fifth enlarged edition. Boston: Thomas Hall, 158 Washington street, 1859. 12mo, pp. 242.-Dr. Channing's excellent little volume has been known for many years by its former editions. It enuinerates a large number of diseases to which electricity in some form has been applied with more or less of success, with quotations of cases. In an appendix, Mr. Hall (successor to D. Davis, Jr.) gives his Illustrated Catalogue of Electromedical Instruments, so favorably known by the medical profession for their superior excellence.

9. Treatise on the various Elements of Stability in the well-proportioned Arch, with numerous Tables of the Ultimate and Actual Thrust; by Captain D. P. WOODBURY, U. S. Corps of Engineers. 438 pp., 8vo, with many plates and tables. New York, 1858: D. Van Nostrand. Being No. 7 of papers on Practical Engineering published by the Engineer Department for the use of the officers of the U. S. Corps of Engineers.

10. Maury's Sailing Directions, Eighth edition, enlarged and improved. 383 pp. 4to, with numerous plates.

A. MALHERBE, of Metz: Monographie des Picidés, ou Histoire générale et particulière de ces oiseaux zygodactyles. This work will extend to 2 vols. fol. of text and ? vols. fol. containing 125 colored lithographic plates and 700 figures. Only 150 copies will be published.

C. F. NAUMANN : Lehrbuch der Geognosie. Second edition, enlarged. 1st volume. xvi and 960 pp. 8vo, with 325 figures. Leipzig, 1858.

Proceedings Boston Soc. Nat. Hist., 1858.-p. 385, On the Species of Flying Fish along the coast of North America; Weinland.-On a new species of Skate, Goniobates, from the Sandwich Islands; Agassiz.-On a new species of Zeus, from Provincetown, Mass. ; Storer.- p. 391, Notice of Dr. J. Deane ; Bouvé.-P. 394, Fossil post-pleiocene Star fish, at Lewiston, Me.; W. W. Baker.-P. 395, Parasites in the American Deer; Wyman.- p. 396, Note on Crustacean Parasites from the Sun fish; kneeland.-Feeding and Growth of the American Robin (Turdus migratorius); Treadwell.-. 400, death of Dr. N. W. Cragin.

PROCEEDINGS Acad. Nat. Sci. PhiladeLPHIA, 1858.-P. 177, Description of a new Tanager, from the Isthmus of Darien, and note on Selenidera spectabilis; John Cassin.-p. 179, Description of a new species of Argyris; J. C. Fisher.- p. 180, Note on the species of Eleodes found in the United States; J. L. Le Conte.

THE

AMERICAN

JOURNAL OF SCIENCE AND ARTS.

[SECOND SERIES.]

ART. XVII.The Atlantic Cable; by GEORGE MATHIOT, Electrotypist of the U. S. Coast Survey.-(In a letter to Prof. A. D. BACHE, Supt. of the U. S. Coast Survey, dated Coast Survey Office, Washington, Sept. 11, 1858).

As the Atlantic Cable has been laid for more than a nionth and the line is not yet opened to the public, it is evident that there is some great difficulty in the way. According to the newspapers the trouble is in the recording instruments. I fear, however, that such is not the true cause of the delay. I investigated the electrical conditions which would obtain in the cable as soon as I had procured a specimen of the material, and ascer. tained the electrical views entertained by the managers. From that investigation I positively predicted that the cable would be a failure unless other views of the generation and distribution of electricity should obtain in the subsequent management.

It is greatly to be regretted that the counsel of certain electri. cians prevailed in the construction of a cable having so small a conductor, and so thin an insulation. But now that it is laid, such as it is, at such immense labor and expense, in conjunction with fortuitous circumstances on the ocean, it becomes us to summon every aid of science and independently of theory to interrogate her experiments and hearken to the infallible directions which these give through the unerring calculus.

Experiment shows that an immense amount of electricity is disposed on the sides of the conductor before the current mani. SECOND SERIES, Vol. XXVII, No. 80.- MARCH., 1859.

fests itself through it. Theorists attribute this delay solely to the circumstance of the electricity being thus disposed, and so leave us without a remedy. The investigations of Ohm, however, tell us that the time of conveying a given quantity of electricity between any two points is determined by the tension at those points and the resistance between, and hence we infer that if the sides of the wire must first be filled before any will pass through, then the time will depend on the quantity required, the tension at the source and the goodness of the conductor. Why should the mere circumstance of where the electricity is disposed cause the delay in its disposal ?

I considered the electrical conditions for a submerged conductor on the first proposal for an ocean telegraph, and was so fortunate as to have a paper containing some of my deductions, published in the Coast Survey report for 1855. In that paper I described a voltaic experiment which proves that a battery requires time to generate the quantity of electricity required to produce the tension needed to overcome a certain resistance, and inferentially, that time is required for the generation as well as for the distribution of electricity. If you will turn to that paper you will there see foreshadowed all the present difficulties of the cable; so fully am I persuaded of the correctness of the principles there given, that I would confidently undertake to charge the cable in a very short time or extend the time of charge many minutes. In the limits of a letter I will not be able fully to lay open the principles which I conclude should govern the electricians of the cable, and fearing to intrude on your time I will proceed rapidly to elucidate the main points and describe what I would propose.

The difficulties of the “static induction" are all prevised by Ohm, and I cannot but wonder that the European electricians did not thence comprehend it; the delay of the current I thence anticipated before experiment had exhibited it. The fundamental principle of Ohm is that the quantity of electricity which passes between two contiguous particles in a given time is proportional to the difference of tension between them. Regardless of this fundamental idea of the distribution, the electricians of the cable have sought to work it as a simple problem of the conduction of a quantity through, as on fully insulated conductors (those suspended in air) instead of considering it as the problem of the charge and discharge of a Leyden jar (an unlimited num. ber of momentary conducting lines): thus regarding the conditions of the flow of the current through the conductor, and ignoring the circumstances under which those conditions are established.

The quantity of electricity which a Leyden arrangement takes as a charge, is proportional to the intensity of the source, the

extent of surface, and the thinness and nature of the insulation, (the dielectric); the time required for charge is determined by the quantity to be conveyed, the resistance of the conductors and the difference of the tensions, and the further resistance due to a certain constant, dependent on the nature of the electric medium, (its coefficient of elasticity,) which is wholly undetermined so far, and probably.can not be handled before we obtain conducting lines of many thousand miles in length, but wbich is so small that it may be disregarded in all practical applications of electricity: in which we may make the expression of the time the ratio of a fraction (

for frection | T

1e+R)"

6) where T, t, are the tensions, Q the quantity required, and R the resistance, and to certainly have the means of increasing or diminishing it to the extent of our ability to handle the generating electrical apparatus, up to the point of fusion of the conductor. By Ohm's principles, and also by the deinonstrated experiments of Coulomb, when a sur. face charged to any given intensity is brought in contact with another surface the electricity is distributed between them and the tension falls in proportion to the combined surfaces divided by the first surface; the time to convey a given quantity of electricity into a given surface will depend therefore on the extent of the first charged surface and the tension of the charge, with the resistance to conduction. As the quantity of electricity

.. Tflowing from S to s at any moment after contact will be and the time for passing the whole quantity or producing equilibrium ITGero) will be dependent on the quantity to be passed, and as this latter is 3, it is evident that the whole time for charge is dependent on the value for S as well as of s and the tension of the source; and that therefore the time in the problem of the conveyance of a given quantity of electricity from a charged surface Sto a recipient surface s will be a decreasing function of S.

The Atlantic cable is a Leyden battery of over four acres of coated surface, (the surface of the conductor). The electrical arrangements for working the cable I cannot learn, but suppose they use an ordinary intensity battery of small pairs, probably having terminal plates of but a few square incles of surface: hence we have the ratio of Sto s as millions almost; when this small charged surface contacts with the great one, its tension is lowered enormously, the difference of tensions is almost de. stroyed, consequently the quantity carried in a given time is proportionally diminished ( =Q; when 1-1=0 then Q=0).

[ocr errors]

Is it any wonder under such circumstances that four or even ten seconds, as rumor states it, is required to convey into the cable the quantity of electricity required to raise the tension to the point due to the great number of elements in the voltaic generator they employ.

The electricians of the cable seem to have concluded that the tension of the terminals of the battery would continue after con. tact, although the demonstration of Obm, and in fact all the analysts of the distribution, have shown that it would fall, and my experiment published in the report of 1855 has even made the decline visible to the senses; or they seem to have proceeded on the supposition that the battery would in all cases maintain a maximum intensity by generating the electricity faster than it could be carried: that is as though the terminals of the battery would maintain the tension exhibited in the open circuit. They appear to have been likewise regardless of the warning of Ohm, that in the case of the exterior conduction resistance being removed or greatly decreased, a certain additional resistance will arise from the mechanical obstruction of the chemical reagents and products, a resistance which must be determined not only for each electro-chemical arrangement but also for each mechanical construction and size. These are conditions however which do not generally affect the distribution, and sensibly affect the generation only when the battery is under circumstances for vigorous action; that such circumstances obtain when the contact is made with the four acres-circumstances which are almost equivalent to uniting the terminals of the battery with a short thick wire—is shown by Mr. Faraday's describing “a rush of electricity into the wire" at the moment of contact (see Phil. Mag. vol. vii, p. 199). In the general formula 1,=Q which is generally taken for the equation of the distribution, if we make R+p=0, Q becomes infinite: a condition which constitutes detonation and readily obtains to the amount of the matter present in the firing of certain mixed gases, guncotton, the fulminates and a few more substances, but which thank heaven is hindered from obtaining in the great majority of electrical actions (the multitudinous phenomena of nature) by the obstruction due to mechanical resistances. One consequence of the removal of the whole of the conduction resistance would be that when we touch a piece of oxydable metal, both the metal and finger should explode; happily the mechanical resistance prevents this, and even in the most energetic forms of the voltaic battery it is so great as to make the generation of very large quantities of electricity a matter of great difficulty.

To form an idea of the limiting power of this resistance I dusted pulverulent platinum over a plate of zinc of one square

« AnteriorContinuar »