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 71/2 hours: one cell
gives an E.M.F. of two volts. In order to have a constant power of one
horse for 71/2 hours, at the rate of 40 amperes discharge, we must have
more than nine cells per electrical horsepower; and 47 such cells will