points out that the name "probable error" is unfortunate, and so we think; he is also of opinion that the average error might with advantage be more used than it is at present as a measure of the precision of a set of observations. This chapter is concluded by a most instructive discussion on the laws of error, based on various assumptions as regards the number of sources of unavoidable error. It is first supposed that there is only one source of error, and that all errors between certain limits are equally probable; the curve of error then becomes a finite straight line. The next case considers two independent sources of error, the curve then becomes two straight lines intersecting on the axis of y at an angle of 45°. In the third case three sources of error are assumed, and the curve of error is shown to consist of three parts, which together form a close approximation to the usual curve of error. The the succeeding three chapters, and applied to method of least squares is further developed in the adjustment of the direct observations of one tions. Various methods of solving the numerunknown, to indirect and to condition observaous resulting equations are given, both rigorous and approximate amongst the latter the tions as used in reducing the primary triangumethod of solution by successive approximalation of the Ordnance Survey of Great Britain is strongly recommended. The author also recommends the use of a calculating machine, or of Crelle's Tables, in order to diminish the arithmetical labor. A TREATISE ON THE ADJUSTMENT OF OBSER-Line VATIONS, WITH APPLICATIONS TO GEO DETIC WORK AND OTHER MEASURES OF PRE CISION. By T. W. WRIGHT, B. A., C. E., late Assistant Engineer United States Lake Survey. New York: D. Van Nostrand. 1884. Price, $400. This treatise will be found a valuable addition to the literature of geodetic operations; the title is, however, misleading-it implies a discussion of the various corrections required to allow for the effects of temperature, refraction, &c. Such corrections, however, are either omitted or only superficially dealt with, and the principal subject matter is the adjustment of unavoidable errors by the method of least squares. The work commences by a discussion of the various causes of error, and several practical hints are given as to how to diminish them. A remark in connection with personal error is worth quoting: "A good observer, having taken all possible precautions with the adjustments of his instruments and knowing no reason for not doing good work, will feel a certain amount of indifference towards the results obtained. The man with a theory to substantiate is rarely a good observer, unless, indeed, he regards his theory as an enemy, and not as a thing to be fondled and petted." In the second chapter the usual law of error is stated, and the method of least squares is deduced therefrom, together with formulæ for calculating the mean square error, the probable error, and the average error. The author The remainder of the work is devoted to applying the foregoing to triangulation, to basemeasurements, to spirit leveling, to trigonometrical leveling, to the graduation of line measures, to the calibration of thermometers, and to the discovery of empirical formulae. The application to triangulation is treated very fully, and several methods of solving the necessary equations are given and illustrated by means of examples. One of these examples is the adjustment of the angles of a quadrilateral taken from the Survey of the Great Lakes of North America, executed by the United States engineers; three methods of solution are given, one of them being that adopted by the United States engineers. It may The author remarks very truly that it is a waste of time applying the rigid methods of adjustment to tertiary or even to secondary triangulation, and he proposes a method of successive approximations by first adjusting the angles at each station for the local conditions, and then using these adjusted values for the further adjustment in connection with the side and angle equations of the net. be mentioned that the reduction of the secondary triangulation of Great Britain, now being carried out, is effected by a graphic method applied after the angles have been locally adjusted; this method is found to give excellent results with far less labor than even an approximate method of calculation. The criticism on the title of the work is well exemplified in the chapters on base-line measurements and on the graduation of line measurements. For instance, there is no mention of the corrections required to be made to a base-line measure ment to allow for errors in alignment or of level, for the effects of temperature and for reduction to sea level. We think that, at any rate, a sketch of these and other sources of error and their methods of adjustment would not have been amiss. The adjustment of the errors of trigonometrical leveling is very fully considered, and one of the examples proposed for solution is the adjustment of the levels taken trigonometrically | during the triangulation executed to determine the axis of the St. Gothard tunnel. The following remark is, we think, worth quoting: "Closely allied to the preceding (elimination of accidental errors) is the common idea that if we have a poor set of observations good results can be derived from them according to the method of least squares, or that if work has been coarsely done such an adjustment will bring out results of a higher grade. A seeming accuracy is obtained in this way, but it is a very misleading one. The method of least squares is no philosopher's stone; it has no power to evolve reliable results from inferior work." An excellent feature in the work is the illustration of the text by means of examples, embracing almost every possible case that occurs in practice. Some of these examples are fully worked out, others are proposed as exercises. Most of them are derived from geodetic work carried out in the United States. In conclusion we can strongly recommend this book.-Nature. MISCELLANEOUS. ESCRIPTIONS of some waterproof varnishes DESCRIPTIONS of some Society of Chemical Industry from the Papier Zeitung as follows:-(1) One part Damar resin; four, five, to six parts acetone are digested in a closed flask for two weeks and the clear solution poured off. To this, four parts of collodion are added, and the whole allowed to clear by standing. (2) Thirty parts white shellac are digested with 500 parts of ether, and to the solution fifteen parts of lead carbonate are added, then shaken for some time and repeatedly filtered. (3) Five parts of glue are dissolved in 100 parts of warm water, and this solution spread on paper. After drying, the paper is soaked for an hour in 10 per cent. solution of acetate of alumina and again dried, in order to give it a final glaze. (4) 120 parts of linseed oil are heated and poured into a mixture of thirty-three parts of quicklime and twenty-two parts of water, to which fifty-five parts of melted caoutchouc have been added, stirring all the time. The varnish is strained and used hot. (5) One part of gutta-percha is carefully digested in forty parts of benzine on the water bath, and the paper covered with it. This varnish can be drawn or written on. T the Montreal Meeting, Prof. Frankland two separate portions, one having an E.M.F. of 2 volts and upwards, the other an E.M.F. of 0.5 volt and under. One of these may be conventionally termed useful, and the other useless, electricity. (2) The proportion of useful electricity obtainable is greatest when the cell is discharged intermittently, and least when the discharge is continuous. (3) Neither in the intermittent nor continuous discharge at high E.M.F. is the current, through uniform resistance, augmented by rest. At low E. M.F., however, the current, after continuous discharge of the high E M.F. portion, is greatly augmented, but only for a few minutes. This augmentation of current at low E.M.F. after rest is hardly perceptible when the high E.M.F. discharge has been taken intermittently. (4) The suddenness of fall in potential indicates two entirely distinct chemical changes, the one resulting in an E.M.F. of about 2.5 volts, the other in one of about 0.3 volt. (5) The chemical change producing low electromotive force is the first to occur in charging, and the last to take place in discharging the cell. It is the change which occurs during what is called the "formation" of a cell, and for economy's sake, a reversal of this change should never be allowed to take place. (6) Currents of enormous strength can be readily obtained from storage batteries coupled up in parallel, viz., a current of 55,000 amperes from only 100 cells. Such a current reduces to insignificance the output of the largest dynamo ever built. It is to be hoped that currents of this magnitude will open up new probabilities of research into the construction of matter. -Engineer. Regenerative accumulator" is formed EGENERATIVE ACCUMULATOR.-M. Zenger's tive pole, with a halogen, such as bromine, by surrounding the electrode forming the posichlorine, or iodine. These halogens serve to depolarize the electrode in combining, when the circuit is closed, with the hydrogen upon the cathode. M. Zenger uses bromine because of its comparative cheapness and fluidity, rendering it preferable to gaseous chlorine and hydrate of chlorine, which are very unstable, or to iodine, which is costly and of a solid consistency. The bromine is placed at the bottom of a porous vase filled with fragments of retort carbon and closed by a covering of paraffin bromine. A layer of chloride of iron is placed furnished with a cork hole to insert the liquid over the carbon; and the zinc plate is surrounded with a solution of dilute chlorhydric acid (1 to 10) mixed with 5 per cent. of glycerine to reduce the resistance of the zinc-carbon bromine cell. A very constant battery for telegraphic work, where the line has a considtrated solution of chloride of iron, more or erable resistance, is obtained from a concenless diluted. In this way a hydro-electric battery of 1.95 volts electromotive force and 0.5 to 5.2 ohms internal resistance is obtained, which gives 3.9 to 0.38 amperes according to A communicated the results of a study of its resistance. The constancy of the pile is the phenomena attending the discharge of ac- considerable; and the cell can be regenerated cumulator cells containing alternate plates of in a short time by the current from a dylead peroxide and spongy lead: (1) The en- namo, which effects the reduction of the ergy of a charged storage-cell is delivered inbromine. VAN NOSTRAND'S ENGINEERING MAGAZINE NO. CXVI.-APRIL, 1885.-VOL. XXXII. HYDRAULIC PROPULSION. BY SYDNEY WALKER BARNABY, Assoc. M. Inst. C. E. So much has been said and written on the subject of hydraulic propulsion, and the principles underlying it are so well understood, that the author proposes, notwithstanding the title of the paper, to give little more than a description of the latest boat propelled by this system, and a comparison of it with a sister-boat driven by a screw, and with some hydraulic vessels built previously. Public attention was first prominently directed to the claims of this propeller by the trial of the "Nautilus" in 1866. This vessel, which was 115 feet long, and had engines of 127 indicated H. P., attained a speed of 8.32 knots per hour. She was propelled by a turbine 7 feet in diameter, which drew water from an opening in the bottom of the boat near the fore part, and discharged it through two nozzles in the sides, just, above the water. The area of each nozzle was 78.54 square inches. The first mention of this system of propulsion which the author is able to find is in an old patent taken out in the year In 1866 also the "Waterwitch," an ar1661 by Toogood and Hayes, for "a par- mored gunboat, 162 feet long, 32 feet ticuler way of Forceing Water through the beam, and having 1,161 tons displaceBottome or Sides of Shipps belowe the ment, was built for the Admiralty at the Surface or Toppe of the Water, which Thames Ironworks, the machinery being may bee of singuler Vse and ease in navi- designed by Ruthven, and constructed by gacon." Many patents were subsequently Messrs. Dudgeon at Millwall. She was obtained, and Mr. Ruthven, whose patent driven by two water-jets, discharged from is dated 1839, built two vessels, one vessel nozzles at the sides level with the water, 9 feet long, which was tried in Edinburgh, the mean diameter of each of which was and the other 40 feet long tried on the 24 inches. The speed attained by the Forth in 1844. A vessel for commercial vessel was 9.3 knots per hour. This ship purposes, fitted with Ruthven's propeller was sister to the "Viper," a twin screwwas built in Prussia in 1853; and, besides ship, but the latter suffered from the disothers, a floating fire-engine was con- advantage of a double keel aft and a structed on the Thames, in which, by the slightly fuller run. A detailed compariadvice of the late I. K. Brunel, the pump-son between the two will be found in ing power was utilized for propelling the Table 1. vessel, an adaptation of the turbine propeller for which it is specially suited. VOL. XXXII.-No. 4-19 In 1878, a hydraulic torpedo vessel was built by the Swedish Government for competition with a similar vessel with the cylinder, works up and down in it. twin screws; and by the kindness of her The cylinder being now full of water, and designer, Mr. Lilliehook, the author is able the float consequently at the top, steam is to give drawings of her arrangements. Figs. 1 and 2 show the position of the pumps, and of the inlets and outlets. In this vessel two pumps were used, not because the designer thought it an advan tage, but in order that a passage might be maintained from the forward to the after part of the ship below the deck. The performance of this vessel will likewise be found in Table 1. admitted by a valve above the float and driving it down, ejects the water through the nozzle. On reaching the bottom of its stroke, the float opens the exhaust, and the steam passes into the condenser. The vacuum thus created in the cylinder causes the water to rise partly through the nozzle, but principally through a suction-valve in the bottom of the condenser. The cylinder is thus filled with water, |