possible, with any reasonable degree of accuracy, to draw the load curve for a proposed sub-station, the question of whether it is more economical to install rotary converters or storage batteries to take care of the peaks may be determined in advance. Most railway load curves show peaks in the morning and evening, some portions of which may be taken by batteries working up to their one-hour rate, provided the duration of the peaks is not too long. In case the shape of the peak is such that no portion of it is of shorter duration than three or four hours, the cost of a battery for handling it would probably be excessive. It should be taken into account that a sub-station battery carrying the peak of the load not only takes the place of a certain amount of rotary capacity, but exercises a valuable smoothingout effect on the load, replacing an equivalent amount of station capacity at the central power plant; so that the investment in battery should be compared with the investment in rotaries, high tension feeders, and in generator and steam plant capacity thus dispensed with at the power plant. Figured at the one-hour discharge rate, the relative first cost of storage battery as compared with rotary converter, feeder and steam plant is approximately $90 per kilowatt for the storage battery and $130 per kilowatt for the converter plant, steam plant and transmission line, the latter figure being $30 for the rotary plant and $100 for the steam plant and transmission line. This estimate for rotary and steam plant is based on the nominal ratings of the machines and does not take into account their overload capacities. A battery worked on the peak at its one-hour rate is likely to cost about the same or somewhat less than the apparatus which it displaces.

The value of the storage battery as a reserve in heavy substation work depends upon the comparative amount of storage battery and rotaries which has been installed. If the sub-station contains a small amount of battery and a large amount of rotary, the reserve value of the battery is likely to be small; if, however, the short duration and sharp nature of the peaks has warranted a comparatively large battery installation, the battery may be of sufficient size to handle the entire load of the sub-station during the lighter hours of the day for a period long enough to tide over a break-down, provided that this period is not longer than half to three-quarters of an hour. While the battery may replace those rotaries which would be operated for an hour or two on the peak, it cannot be depended upon to take the place of those machines which run throughout the entire day. The reserve feature of a storage battery is especially valuable because it may be put into service immediately and in this it has the advantage of the steam plant, where time is required to get boilers ready for steaming.

In laying out a sub-station, it would be well to design the building with reference to the future installation of storage batteries, even

No general statement

if it is not intended to install them at once. can be made in regard to the advisability of their installation, as that should be figured out for each special case.

Low Tension Switchboard.

The feeder board for the sub-station and the 500-volt feeders leading from it to the trolley sections are practically the same as those used in connection with a direct current station, and all of the devices installed in the best direct current stations for the detection and isolation of trouble on the trolley sections should also be placed in the sub-stations.

Sub-Station Plans.

Figures 4A, 4B and 4C show the plan and cross-sections of the sub-stations of the Manhattan Railway. These buildings were designed for the purpose of sub-stations and represent the best practice in sub-stations for heavy work. Unfortunately, in many recent installations, sub-station machinery has been stuck into any out-of-theway place, and a good sub-station design is comparatively rare.

THE RELATIVE ADVANTAGES OF ALTERNATING AND DIRECT CURRENT GEN

ERATION AND TRANSMISSION.

With a desire of learning exactly what are the conditions in the cities of the United States, a circular letter was mailed to roads in all cities of more than 100,000 inhabitants. The information is tabulated in Table 3 for those railways from which answers were received. It will be observed that the use of alternating current for city work is at present largely confined to the operation of suburban roads from city power stations. With the exception of New York, most of the other cities have a mixed system, direct current being used for city work and alternating current for suburban work. Brooklyn and Philadelphia are now building alternating current stations and Chicago will probably soon begin the change to alternating current distribution. With the single exception of New York, all cities of more than 200,000 inhabitants have from three to eight power stations. Whether this condition is more economical or more desirable than distribution from a single power station is a problem which must be solved for each special case. The number of American roads using alternating current for purely city work is at present very small, but it is likely that within the next few years many city roads will change partially, at least, to the alternating current system.

In Table 4 is tabulated general data regarding the alternating current installations in the large cities. It is difficult to tabulate such data because special conditions exist in nearly every case. It will be noted that three-phase distribution and a periodicity of twenty-five cycles per second is nearly universal.