are given in the following table, which is based, partly on figures given by Mr J. F. C. Snell, and partly on the Author's experience :

The quantity of water required for condensing is obviously very great, but the saving in feed water effected may quite balance this in many cases. Where a plentiful supply of water cannot be had, some method of cooling and re-using the water must be adopted.

The oldest and simplest method is to return the water to a pond, and allow the water to part with its heat naturally to the air. Cooling takes place by radiation, conduction, convection, and (chiefly) by evaporation.

The rate of cooling by radiation will depend on the excess temperature of the water above that of the air, and the clearness of the atmosphere; dissipation of heat by conduction and convection will depend upon the wind; the amount got rid of by evaporating the water will depend on the dew point of the air, the atmospheric pressure, the temperature of the air and of the water, and the removal of the air as it becomes saturated.

Obviously, what is required is a pond having as great an area as possible, while the capacity of the reservoir must be such as to enable a sufficient quantity to be stored to allow the cooling time to take place. The area of surface should be about 2 square feet, and the capacity about 10 cubic feet, per pound of steam coudensed per hour.

The cost of this system of cooling is very high, and, where land is dear, prohibitive. For a concrete pond alone, without allowing for the cost of land, the cost of construction may be taken at about 3s. 9d. per pound of steam condensed per hour. The cost is, however, wholly one of interest charges, since the water returns to the pond by gravitation, and the cost of upkeep is practically nil.

Considerable economy of space may be effected by pumping the water at a pressure of from 10 to 20 lbs. per square inch into a series of pipes, whence it escapes through nozzles in the form of fine spray, falling then into a pond. By this means a very large surface is exposed to the air, thus promoting evaporation, but the air rapidly becomes saturated, and so full advantage cannot be taken of this increased surface, while radiation is hindered by the clouds of vapour formed.

L

The next improvement is to pump the water to a height of from 15 to 20 feet, and allow it to trickle down over a number of plates, sometimes consisting of sheets of corrugated iron, sometimes of wooden strips placed vertically, alternate sets being at right angles to one another. By these means the passage of water is very slow, and the heat it parts with induces a circulation of air through the apparatus. The floor space occupied is only about onetenth of a square foot per pound of steam condensed per hour.

All these systems necessarily cause a large amount of spray and vapour to be discharged into the air, and hence may cause considerable nuisance. In order to avoid this, and to reduce the floor space to the smallest possible amount, the last-named method has been developed into the cooling tower, in which the vapour is discharged from the top of a tall tower, like smoke from a chimney, and a powerful draught is often maintained by means of fans.

One form of such cooler consists of a number of rectangular laths placed horizontally and transversely within an enclosing chimney up which the heat of the water causes a draught. The water to be cooled is pumped into a trough, about 23 feet above the ground, whence it falls through the cooler. This cooler occupies less than one-twentieth of a square foot of floor space per pound of steam condensed per hour.

When a fan is used to increase the draught, the tower can be made much higher, and the floor space correspondingly reduced, less than onehundredth of a square foot sufficing for each pound of steam condensed per hour.

The cooling and distributing surface is, in some cases, formed of wood, enclosed in a wooden chimney; in others of earthenware; and in others, again, of metal gauze, the containing vessel in these two latter cases being of wrought iron.

The fan is usually placed at the base of the tower, but sometimes at the top, in which case the draught is an induced one.

The power taken to drive the fans will be about 3 per cent. of that developed by the engine, the steam from which is being dealt with; while the first cost of a cooling tower with fan draught may be taken as approximately one shilling per pound of steam condensed per hour. As regards cost of upkeep, contrary to what one would expect, the wooden apparatus is found to be durable, the reasons assigued for this being free access of air to the material, and, when jet or ejector condensers are used, the deposit of oil on the surface.

The amount of water lost by evaporation is about 2 per cent. of the circulating water.

An incidental advantage of these cooling arrangements is that the water becomes deprived of most of its air, thus considerably lessening the work to be done by the air-pump.

CHAPTER XVII.

SWITCHING GEAR, INSTRUMENTS, AND ELECTRICAL

CONNECTIONS.

THE term 'switching gear' covers the whole of the apparatus and devices for controlling and regulating the supply, as well as those for actually directing the current into certain mains, which is presumably its original significance, derived from the analogy of railway practice.

The switchboard is the most vital part of the whole system of supply, for to it converge all the internal work connected with the generating plant and all the external system of mains for feeding and distributing. It may be likened to the spinal cord, through which all the impulses travel, and which, if damaged, disables the entire organism. An electric supply system may survive the loss of a member, whether it be a generator or a main; but if its switchboard break down, it is at once hors de combat.

The primary object of a switchboard is to gather the current generated by the dynamo machines, and direct it, as desired, into the underground mains that convey it to the points of utilisation. Incidental to this object is the control of the energy, and the protection of the generators and mains from damage when abnormal conditions arise. The second main function of the switchboard is the control of the pressure of supply and the appliances for enabling machines to be suitably brought into service. Thirdly, the instruments necessary for the measurement of the output of the generating plant, the indication of the power supplied to the mains and its mode of distribution over the system, together with the measurement of the pressure at which it is supplied, form an integral part of the switchboard.

Switchboards fall broadly into two classes, viz.: (a) those intended for high pressures and moderately large currents; and (b) those for exceedingly large currents at low pressure. These two sets of conditions require very different treatment in some respects, but certain points in their design are common to both.

The essentials of all switchboards are:

(1) The whole of the apparatus must be composed of incombustible materials.

(2) The disposition of the apparatus must be such that there shall be no danger to life, whether from shock, fire, or mechanical force, during its normal or abnormal working.

(3) Every part must be so calculated that it shall be free from heating when used continuously.

(4) The scheme of the board must be such that the purpose of every portion of it shall be self-evident, without reference to diagram or description; and, in some cases, it is desirable that the various parts shall be so locked mechanically or electrically that it is impossible for the various operations to be accidentally performed in the wrong order.

(5) All the working parts must be accessible for cleaning, and must be so arranged that they can be handled safely.

(6) Portions subject to wear must be easily renewable.

(7) In most cases, it is necessary that the board should be so arranged that it can be readily extended to deal with increased output without alteration or disfigurement.

The incombustibility of the switching gear is placed first, because it is, perhaps, the most important essential of all. There is a tendency to study appearance at the expense of this requirement, and to ornament the board with pitchpine or other wooden panelling; especially is this the case in Continental stations. Happily this practice is rapidly being abandoned in this country; but the boards that are coming in leave much to be desired, for to appearance is still assigned too much importance, handsome marble slabs being used to give a fine front, and, like whited sepulchres, to hide what is too often a chaotic mass of cables, insulated with such a highly inflammable material as indiarubber.

It ought to be recognised that, for a large central station, a switchboard is just as much a piece of mechanism as any other part of the plant, and should be treated as such. No one would think of boxing round a governor with polished pine, nor of lacquering the arms of a fly-wheel, and this class of work is just as much out of place on a switchboard. Its beauty should be that of design and fitness for the purpose it has to serve, not of adventitious ornament. On these principles, the present passion for marble panels is to be condemned; they are out of place in au engine room, they serve no purpose that could not be better fulfilled in other ways, they are expensive, and, finally, they soon become discoloured and cannot be readily cleaned. It is often contended that marble is better than slate; it certainly is in most respects, but slate is itself most unreliable, and its use should be avoided as far as possible.

A switchboard should consist as wholly of metal as possible. If for high pressure work, every particle of metal not forming part of the circuit should be connected efficiently to earth; and especially should all levers, and other parts that have to be handled, be most carefully kept at earth

potential In high pressure work there is no half-way house between perfect insulation and earth, any partially insulated objects are liable to become charged to dangerous potentials, and especially is this the case with high pressure continuous current.

Although the switchboard framework and the platform carrying it should, for high pressure work, be carefully earthed, it is well for the attendants to be themselves insulated by means of rubber mats or rubber shoes, since, under no circumstances, should they form a conductor to earth. On the other hand, no reliance should ever be placed on this insulation by venturing to handle live objects. The Author has known insulated stools provided, and instruments carrying pressures of over 2000 volts handled; but such a practice is most reprehensible, so simple a matter as an uninsulated person handing a tool to one on a stool being sufficient to cause the death of both persons.

In the case of low pressure boards, an entirely different practice is desirable as regards earthing. Here everything that can be done to help the insulation should be resorted to, and, for low pressures, very moderate amounts of insulation are effectual, while dangerous charging cannot take place. There is not the necessity for keeping everything at maximum stress, and by resorting to double insulation the construction is greatly simplified and cheapened.

Switchboards ought to be so placed that access can be had all round them; the back should be as open as the front. They should be placed in such a position that they are unlikely to be affected in the event of a disaster at the station, and hence should be well away from all running machinery, and quite clear of steam or water pipes.

In large stations, it may be advisable to have separate boards for the generators and mains, the switches for connecting and disconnecting the generators being alone placed in the station, and the switches controlling the mains being grouped together in a distinct room. This is especially the case in a large high pressure station, the engine-room man having quite enough to do to look after synchronising and switching his machines in and out, without troubling about the feeders.

Switchboards in an engine room should be placed in such a position that they command a view of the generators; and with this end in view, they are usually mounted on an elevated platform. This should preferably be of steel, with steel floor-plates for high pressure work, or with a concrete floor, tiled, for low pressure boards.

In calculating the carrying capacity of the various parts of the switchboard that carry the current, it must be remembered that the temperature in an engine room is usually high, and the board must remain as cool as possible, otherwise switches may jam, or fuses may melt below their proper temperature.