From a recent number of the Annales de l'Electricite, we learn that Count de Moulins experimented on the lake in the Bois de Boulogne, in the year 1866, with an iron flat-bottomed boat, carrying twelve persons. Twenty Bunsen cells furnished the current to a motor on Froment's principle turning a pair of paddle wheels.

In all these reports there is a lack of data. We are interested to know what power the motors developed, the time and speed, as well as dimensions and weights.

Until Trouve's trip on the Seine, in 1881, and the launch of the Electricity on the Thames, in 1882, very little was known concerning the history of electric navigation.

M. Trouve originally employed Plante's secondary battery, but afterward reverted to a bichromate battery of his own invention. In all the primary batteries hitherto applied with advantage, zinc has been used as the acting material. Where much power is required, the consumption of zinc amounts to a formidable item; it costs, in quantity, about 3d. per pound, and in a well arranged battery a definite quantity of zinc is transformed. The final effect of this transformation manifests itself in electrical energy, amounting to about 746 watts, or one electrical horse power for every two pounds of this metal consumed per hour. The cost of the exciting fluid varies, however, considerably; it may be a solution of salts, or it may be dilute acid. Considering the zinc by itself, the expense for five electrical or four mechanical horse power through an efficient motor, in a small launch, would be 2s. 6d. per hour. Many persons would willingly sacrifice 2s. 6d per hour for the convenience, but a great item connected with the employment of zinc batteries is in the exciting fluid, and the trouble of preparing the zinc plates frequently. The process of cleaning, amalgamating and refilling is so tedious, that the use of primary batteries for locomotive purposes is extremely limited.

To recharge a Bunsen, Grove, or bichromate battery, capable of giving six or seven hours' work at the rate of five electrical horse power, would involve a good day's work for one man; no doubt he would consider himself entitled to a full day's wages, with the best appliances to assist him in the operation.

Several improved primary batteries have recently been brought out, which promise economical results. If the residual compound of zinc can be utilized, and sold at a good price, then the cost of such motive power may be reduced in proportion to the value of those by-products.

For the purpose of comparison, let us now employ the man who would otherwise clean and prepare the primary cells, at engine driving. We let him attend to a six horse power steam engine, boiler, and dynamo machine for charging 50 accumulators, each of a capacity of 370 ampere hours, or one horse power hour. The consumption of fuel will probably amount to 40 lb. per hour, which, at the rate of 18s. a ton, will give an expenditure of nearly 4d. per hour. The energy derived from coal in the accumulator costs, in the case of a supply of five electrical horse power for seven hours, 2s. 9d.; the energy derived from the zinc in a primary battery, supplying five electrical horse power for seven hours, would cost 17s. 3d.

It is hardly probable that any one would lay down a complete plant, consisting of a steam or gas engine and dynamo, for the sole purpose of charging the boat cells, unless such a boat were in almost daily use, or unless several boats were to be supplied with electrical power from one station. In order that electric launches may prove useful, it will be desirable that charging stations should be established, and on many of the British and Irish rivers and lakes there is abundance of motive power, in the shape of steam or gas engines, or water-wheels.

A system of hiring accumulators ready for use may, perhaps, best satisfy the conditions imposed in the case of pleasure launches.

It is difficult to compile comparative tables showing the relative expenses for running steam launches, electric launches with secondary batteries, and electric launches with primary zinc batteries; but I have roughly calculated that, for a launch having accommodation for a definite number of passengers, the total costs are as 1, 2.5, and 12 respectively, steam being lowest and zinc batteries highest.

The accumulators are, in this case, charged by a small high pressure steam engine, and a very large margin for depreciation and interest on plant is added. The launch taken for this comparison must run during 2,000 hours in the year, and be principally employed in a regular passenger service, police and harbor duties, postal service on the lakes and rivers of foreign countries, and the like.

The subject of secondary batteries has been so ably treated by Professor Silvanus Thompson and Dr. Oliver Lodge, in this room, that I should vainly attempt to give you a more complete idea of their nature. The improvements which are being made from time to time mostly concern mechanical details, and although important, a description will scarcely prove interesting.

A complete Faure-Sellon-Volckmar cell, such as is used in the existing electric launches, is here on the table; this box weighs, when ready for use, 56 lb.; and it stores energy equal to one horse power for one hour=1,980,000 foot pounds, or about one horse power per minute for each pound weight of material. It is not advantageous to withdraw the whole amount of energy put in; although its charging capacity is as much as 370 ampere hours, we do not use more than 80 per cent., or 300 ampere hours; hence, if we discharge these accumulators at the rate of 40 amperes, we obtain an almost constant current for 7½ hours: one cell gives an E.M.F. of two volts. In order to have a constant power of one horse for 7½ hours, at the rate of 40 amperes discharge, we must have more than nine cells per electrical horsepower; and 47 such cells will supply five electrical horse power for the time stated, and these 47 cells will weigh 2,633 lb.