66 



SCIENCE. 



[Vol. XII. No. 2[ 



existing knowledge of the prevailing weather conditions in that 

 square. It therefore happens that there are many squares whose 

 meteorological conditions are very well known, on account of the 

 very great number of vessels which traverse them ; while, on the 

 contrary, there are other squares which lie off the tracks of com- 

 merce whose meteorological conditions are only approximately 

 known. In addition to this graphic representation of the frequency 

 and force of prevailing winds in each ocean square, there is printed 

 a brief forecast and a table showing the normal reading of the 

 barometer, arranged in tabular form by ocean squares. A double 

 dotted line near Newfoundland shows the probable limit of the 

 region of frequent fogs for the coming month, and dotted lines 

 across the lower parts of the chart indicate the limits of the trade- 

 winds. Where the north-east and south-east trade-winds meet, 

 there is the region of equatorial rains, indicated on the chart by a 

 blue belt of irregular shape, lying principally north of the equator. 

 These constitute the blue data or portions of the chart. 



The portions of the chart printed in red comprise information 

 collected during the month preceding the date of issue. On the 

 ocean are plotted the latest reported positions of derelict vessels, 

 wrecks and drifting buoys. Dotted lines indicate the drift which 

 each wreck has followed since it was first reported. There are also 

 plotted the positions where whales and waterspouts were reported 

 during the previous month, and a red belt off Newfoundland indi- 

 cates the region where frequent fogs were encountered. In the 

 lower right-hand corner is printed a brief weather review of the 

 preceding month, written at the last moment before going to press, 

 but necessarily more or less incomplete so far as the entire Atlantic 

 is concerned. Above is a large amount of printed matter, compris- 

 ing a list of notices to mariners issued during the previous month, 

 dangerous obstructions to navigation along the coast, charts pub- 

 lished and cancelled, transatlantic steamship and sailing routes, the 

 latest reported positions of logs from the big lumber raft which was 

 abandoned off Nantucket, and various other matter likely to be of 

 timely interest. To one who is not familiar with the subject it 

 would seem almost impossible to publish on one chart such a vari- 

 ety of information of such a diverse character, and yet have a chart 

 that can be of practical use in plotting a vessel's track. It would 

 be very difficult to do without the distinction of colors. 



In describing the methods by which the data for the Pilot Chart 

 are collected from masters of vessels, Mr. Hayden referred to the 

 branch hydrographic offices established in Boston, New York, Phil- 

 adelphia, Baltimore, New Orleans, and San Francisco. At these 

 offices masters of vessels can find all the latest nautical information 

 — charts, light-lists, sailing directions — for every ocean of the 

 globe, and standard barometers and thermometers for purposes 

 of comparison. The naval officer in charge of such a branch 

 office, during his three-years' tour of shore duty, is thrown into 

 intimate relations with the owners, agents, and especially with the 

 practical and energetic masters, of merchant vessels of every de- 

 scription, to mutual advantage, and to the benefit of both the com- 

 mercial marine and the naval service. Mr. Hayden referred for 

 illustration to the working of the branch office established in the 

 Maritime Exchange, New York, which Lieut. V. L. Cottman, 

 U.S.N., during the few years he has been in charge, has brought 

 into a position of usefulness commensurate with the vast shipping 

 interests of the great commercial metropolis of the United States. 

 In a single year (1886-87), 6,739 vessels were visited, nautical in- 

 formation furnished to 83,345 masters of vessels and others, 10,397 

 Pilot Charts distributed, and 3,601 special detailed reports of ma- 

 rine meteorology forwarded for use in the preparation of the Pilot 

 Chart alone, in addition to all the regular office-work, of which 

 this is but a small fraction. 



ELECTRICAL SCIENCE. 



Change of Potential in a Voltaic Couple by Variation of 



Strength of the Liquid. 



Dr. G. Gore, F.R.S., read before the Royal Society, June 14, a 

 communication on the above subject. A voltaic couple, consisting 

 of zinc and platinum in distilled water, was opposed to a thermo- 

 electric pile, the latter being regulated until there was no deflection 

 of a galvanometer in the circuit. To the distilled water there was 



added potassic chlorate, potassic chloride, hydrochloric acid, or 

 bromine, in gradually increasing quantities, and the change in the 

 electro-motive force of the voltaic couple was measured in each 

 case. The following are the minimum proportions of the above 

 substances required to change the potential of the couple in water : 

 potassic chlorate, between i in 221 and i in 258 parts of water; 

 potassic chloride, between i in 695,067 and 1,390,134 ; hydrochloric 

 acid, between I in 9,300,000 and 9,388,188; of bromine, between 

 I in 77,500,000 and 84,545,000 parts. With each of these sub- 

 stances a gradual and uniform increase of the strength of the solu- 

 tion from the weakest to a saturated solution was attended by a 

 more or less irregular change of electro-motive force. 



By plotting the results in curves, — the quantities of dissolved sub- 

 stance as ordinates, the electro-motive forces as abscissse, — each 

 substance will yield a different curve, the form of which is charac- 

 teristic of the substance. 



As a very slight addition of a foreign substance greatly changes 

 the • minimum point,' and alters the curve of variation of potential, 

 the two may probably be used as tests of the chemical composition 

 of the substance, and as a means of examining its state of combina- 

 tion when dissolved. 



The ' Minimum Point ' of Change of Potential of a 

 Voltaic Couple. — Dr. Gore, at the same meeting of the Royal 

 Society, described experiments made to determine the minimum 

 amount of any substance that would affect the electro-motive force 

 of a voltaic cell. To do this he arranged two magnesium-platinum 

 couples in distilled water, and opposed them to each other with a 

 sensitive galvanometer in their circuit. He then added known 

 quantities of the substances to be investigated to one of the cells, 

 and noted when the balance between the two couples was upset. The 

 results were as follows : potassic chloride, between i part in 3.875 and 

 4,650 parts of water ; potassic chlorate, between i in 4,650 and 5,166 ; 

 hydrochloric acid, between I in 516,666 and 664,285 ; chlorine, be- 

 tween I in 15,656,500,000 and 19,565,210,000. The proportion re- 

 quired of each of these different substances is dependent upon very 

 simple conditions, — unchanged composition of the voltaic couple, 

 uniform temperature, and the employment of the same galvanom- 

 eter. If a more sensitive galvanometer was employed, of course 

 the numbers would be increased, but they are relatively correct. 

 With constant conditions, the numbers obtained may possibly be 

 used to test the purity or the uniformity of composition of the 

 dissolved substances. The ' minimum point ' varies with (i) the 

 chemical composition of the liquid ; (2) the kind of positive metal ; 

 (3) to a less degree with the kind of negative metal ; (4) the temper- 

 ature at the surface of the positive metal, and that of the negative 

 one ; (5) with the galvanometer used. The degree of sensitive- 

 ness is related to the degree of free chemical energy of the 

 liquid, also to the atomic and molecular weights of the dissolved 

 substances. The greater {he degree of the free chemical 

 energy of the dissolved substance, and the greater its action 

 upon the positive metal, the smaller the proportion of it re- 

 quired to change the potential. As the ' minimum point ' of a sub- 

 stance dissolved in water is usually much altered by adding almost 

 any soluble substance to the mixture, measurements of that point 

 in a number of liquids at a given temperature, with the same voltaic 

 pair and galvanometer, will probably throw some light upon the 

 degree of chemical freedom of substances dissolved in water. 



Electrical Treatment of Zinc and its Ores. — Mr. 

 Alexander Watt has brought forward a process of purifying and 

 reducing zinc that promises to be largely used. In the purifying 

 process the zinc is made the anode in a bath containing an organic 

 acid, and is dissolved and deposited upon the cathode. Acetic 

 acid is generally used in the process, the ordinary commercial acid 

 being mixed with water in the proportion of one to two. The im- 

 pure zinc plates are suspended in the bath, and the pure zinc is de- 

 posited on thin zinc plates, or on copper or iron plates coated 

 with plumbago. When the operation is finished, the cathode plates 

 are washed, and melted into ingots. To reduce the ores of zinc, 

 especially the carbonate, the minerals are first reduced to a powder, 

 and then submitted to the action of the acid, being added a little at 

 a time. When the zinc is completely dissolved, the liquid is allowed 

 to stand, and is then drawn off, and mixed with water in equal pro- 



