caused by the machinery. Brick or concrete foundations should be finished with a cap or slab of bluestone. This tends to hold the foundations together, and also forms a level surface upon which to set the machinery. If the engine is self-contained, that is, provided with a cast-iron base, a stone cap for brick foundations may be dispensed with.

Walls. The walls of the building may consist of either wood, brick, stone, or iron, depending upon the location and size of the building, and other circumstances. The walls of a station in a small town may be of wood, as already stated; but brick is preferable for many reasons. If wooden walls are adopted, they may properly consist of vertical posts 8 inches square or larger, and 10 feet between centers. These support the roof timbers and wall proper. The latter consists of spruce plank 3 inches thick, grooved and splined, nailed horizontally against the posts. Outside of this one-inch boarding is laid vertically, and the cracks battened with strips of wood; or the outside of the wall may be covered with clapboards.

Fig. 8 shows the standard type of electric-light station recommended by the Boston Manufacturers' Mutual Fire Insurance Co., and designed by Mr. C. J. H. Woodbury. The right-hand side shows a wooden wall, and the left-hand side a brick wall construction. If the walls be constructed of brick, they should be at least 12 inches thick, even in small stations, and 16 or 20 inches thick in large stations. There should be a pilaster at each roof truss, having 8 inches projection and 24 inch face.

The bricks used should be hard burned, and have clean, sharp edges, no salmon or light-colored brick being allowed. The common size of bricks in Eastern cities of the United States is 8 by 4 by 2 inches, equal to 66 cubic inches, weighing about 4 lbs., or 2 tons per thousand. A pressed brick of the same size will average about 5 lbs. each. The crushing strength of bricks varies greatly. A soft one will crush at about 500 or 1,000 lbs. per square inch, while a first-rate machine-pressed brick will not crush with less than 3,000 to 5,000 lbs. per square inch. Cracking and splitting, however, usually commence at about onehalf the crushing load; and to be really safe the load should not exceed one-tenth of the crushing strength. Bricks may be laid in common lime mortar or cement mortar; the latter is much prefer

able, particularly if the walls are subjected to vibration, or are
required to carry considerable weight, in cases where machinery
is put upon floors supported by the walls, or where traveling cranes
are used. Common mortar consists of one part of quicklime and
three to four parts of sand by bulk. About 20 cubic feet of
mortar are sufficient to lay a thousand brick with coarse joints.
of inch, usual in interior walls. In such cases one thousand
brick make 2 cubic yards of massive work, nearly one-third of
the volume being mortar. For outside or other joints which show,

a whiter and thinner layer of mortar is used, made of one part
of lime to 2 or 3 parts of sand. It is necessary to protect
quicklime from moisture, as even the moisture of the air will
cause it to undergo the process of air-slaking.
The average

weight of common hardened mortar is 105 to 115 lbs. per cubic
foot. The crushing strength of good common mortar six months
old is from 125 to 200 lbs. per square inch. Both the sand and
lime of lime mortar should be free from clay and soil. Mortar
should not be mixed upon the surface of clay ground; but a rough

Natural
Level

Pit sand

board, brick, or stone platform should be interposed. sifted is excellent for mortar. Its sharp angles make with the lime a more coherent mass than the rounded grains of river or sea sand, the latter also having the objection of containing salt, which is very difficult to remove. One barrel of unslaked lime (230 lbs.) will make about one cubic yard of ordinary mortar. Mortar should be applied wetter in hot than in cold weather.

As already stated, cement mortar is preferable to common mortar, when required to stand considerable weight or vibration. This consists of 1 part of cement and 2 to 4 parts of sand. It is very important that the cement and sand be thoroughly mixed.

A bricklayer and a laborer to keep him well supplied with materials will, in common house walls, lay an average of about 1,200 to 1,500 brick per day of 10 working hours; in good, ordinary street fronts 700 to 1,000, and on very fine work with angles, etc., 150 to 300. In plain massive engineering work he should average about 1,500 per day. Higher figures than these are sometimes given by engineering authorities, but it is doubtful if they can be realized. This may partly be accounted for by the fact that the working-day is now only 9 or even 8 hours, instead of 10, which was formerly the rule.

Stone walls for stations are used in large cities where more substantial and ornamental structures are desired. The kind of stone employed for the purpose will depend upon what is most available, but would ordinarily be sandstone, limestone, or granite, laid in cement mortar. The cost of good stone masonry of course varies greatly, but is usually between $20 and $40 per cubic yard.

Iron would not be a particularly suitable material for the walls of a central station, because it would transmit too readily the heat and cold; it would make the supporting and insulation of the wires, switches, etc., difficult; and would act as a sounding board for noise and vibration. It would have the advantage, however, of being absolutely fireproof. If iron were used, it would be applied in the form of cast plates or rolled sheet iron, which latter would ordinarily be corrugated.

Roofs. The roof beams, or trusses which support the roof proper, may consist of either wood or iron, the former having the advantage of cheapness, the latter being stronger and fireproof.

The simple roof construction shown in Fig. 8, consisting of rafters 10 or 12 inches square and 10 feet between centers, will answer for small stations. For larger stations regular roof trusses of any of the well-known forms should be used. Iron trusses may also be employed. In any case it is desirable to have a louver or monitor, with side ventilators on the roof, as represented in Fig. 8. Whether the roof trusses be of wood or iron, the roof itself may be made of 3-inch plank splined, having a proper pitch, and covered with slate, tin, or tar and gravel.

In many instances, stations have been provided with roofs entirely constructed of iron. Fig. 9 shows one of a number of iron roofs specially built for electric-light stations by the Berlin Iron Bridge Co. of Connecticut. These are fireproof and neat in appearance, but they require a ceiling or lining of some suitable material, to prevent water from being condensed on the roof in cold weather and dripping upon the dynamos, which would be very objectionable. Several methods of overcoming this difficulty have been devised, some of which are given in the Electrical Engineer (N.Y.) of Jan. 20, and March 9, 1892.

Floors. The engine and dynamo room floor should consist of two layers of plank, the first of yellow or Norway pine splined, and the second of inch maple. The boards of the second floor should not be over 4 inches in width, and should be blind nailed.

A brick or cement floor is very undesirable for a room containing machinery, for the reason that the grit produced by wear is stirred up by walking or sweeping, thereby getting into the bearings and other parts of the machinery, and causing them to wear and heat. Some form of wooden surface for the floor should be provided in rooms containing running machinery. In the case of dynamos generating high-tension currents, another important reason for using wooden, and not brick or cement, floors is the fact that the latter would be apt to cause a man standing on them to have a good electrical connection with the ground, and accidental contact with a dynamo or wire might injure or even kill him. In fact, where currents of over 500 volts are generated, some special means should be carefully provided for securing perfect insulation of the floor in the neighborhood of dynamos, switchboards, and other places where men have to handle the high-tension apparatus. This floor may consist of thick boards or planks having the pores filled with oil, paraffine, or other substance to prevent absorption of moisture. These planks should be held by blind nailing, or by driving the nail heads below the surface three quarters of an inch or more, the holes being filled afterwards with wooden plugs, in order that persons may not, by accidental contact with the nails, be hurt. Where extremely high voltages of several thousand volts are employed, a special insulated floor mounted on glass or other form of insulators should be constructed. A well insulated floor is the best safeguard against shocks to those working about high-tension machinery, and is well worth putting in, although it is not an absolute preventive of accident.

The floor of the boiler-room should be made of brick, concrete, or cement, in order to be fireproof, as there is not the same objection to grit and electrical conduction which makes these materials unsuitable for the floors of the engine and dynamo rooms.

Division of Station Building. In the station building space must be provided for the various parts of the plant and business as follows: The boiler-house may be a separate building, which in some respects is desirable to remove fire risk and dirt. In any case a brick wall or partition should be interposed between the boiler-room and the other parts of the building, in order to shut off danger of fire; and this partition should be impervious without