that spongy lead.  Therefore the depreciation of the battery is almost entirely due to the oxidation of the positive plates.  If we were to make a lead battery of plates 1/4 inch thick, it would last many years; but for street car work that would be far too heavy.  Therefore we make the positive plates a little more than one-eighth of an inch thick.  I find that the plates get sufficiently brittle to almost fall to pieces after the car has run fifteen hours a day for six months.  The plates then have to be renewed.  But this renewal does not mean the throwing away of the plates.  The weight is the same as before, because no consumption of material takes place.  We take out peroxide of lead instead of red lead.  That peroxide, if converted, produces 70 per cent. of metallic lead, so that there is a loss of 30 per cent. in value.  Then comes the question of the manufacture of these positive plates, which, I believe, at the present day are rather expensive.  But I believe the time will come when battery plates will be manufactured like shoe nails, and the process of renewing the positive plates will be a very cheap one.

I ascertained in Europe that the motive power costs 2 cents per car mile; that is, the steam power and attendance for charging the batteries.  We have to allow twice as much for the depreciation of a battery at the present high rate at which we have to pay for the battery—­$12 for each cell.  But I believe that as soon as the storage battery industry is sufficiently extended, the total cost for propelling these cars will not be more than six cents a mile, or about one half the cost of the cheapest horse traction.

I have made some very careful observations on the cable tramway in Philadelphia, which is quite an extensive system.  I have never been able to ascertain the exact amount of waste in pulling the cable itself; but I have it on the authority of certain technical papers that there is a waste of about eighty per cent.  I do not intend to depreciate cable or any other tramways, but there is a difficulty about introducing cable tramways.  It is necessary to dig up the streets and interfere with the roadways.  I have been told that the cable arrangements in Philadelphia cost $100,000 a mile, and that the cable road in San Francisco cost more than that.  One of the directors of the cable company in Philadelphia told me that if he had seen the battery system before the introduction of the cable, he would probably have made up his mind in favor of the former.  The wear and tear in the case of the storage system is also considerable.  There is a waste of energy in the dynamo; secondly, in the accumulator charged by that dynamo; thirdly, in the motor which is driven by the accumulator; and fourthly, in the gearing that reduces the speed of the motor to the speed required by the car axles.  It would be difficult to make a motor run at the rate of eighty revolutions per minute, which is the number of revolutions of the street car axle when running at the rate of ten miles an hour.  Take all these wastes, and you find in practice that you do not utilize more than 40 per cent. of the energy given by the steam engine.  But this is quite sufficient to make this system much cheaper than horse traction.