largely vitiated by the effect of surges. The purpose of the compensator is to immediately set the governor in operation approximately to the full change of gate opening which will be required for a given change of speed or load without waiting until the full change of load or speed occurs. Governors with compensators have been developed to such a degree of perfection that they will anticipate with but small error the total effect of a sudden change in load on the generator almost the instant that the change in load and resulting change in speed begins to take place. Slight errors in the first adjustment are quickly corrected by successive smaller movements of the governor mechanism, as the gate openings become adjusted to the new load conditions

Pondage and storage may be distinguished in that the former is derived directly from a pond formed by the dam supplying the power plant. It is usually used only for the purpose of supplying a quantity of water greater than the average flow of the stream for the day, during hours when the load or demand for power is maximum. The pond is then allowed to refill during hours of the day when the supply exceeds the demand. When such a pond is drawn down to any degree to supply power, the available head is accordingly reduced, and there is generally an economic limit of the amount of the draft from pondage, since the drawing down of the pond to a greater extent reduces the head to such a degree that the power output is not increased by any further

Fig. 6 illustrates a typical water wheel governor or actuator of the oil pressure relay compensating type. Such an apparatus is almost human in its operation, in apparently anticipating pending load changes, the moment they begin, and correcting the gate opening in advance of the complete load change so as to maintain a constant speed.

Water power which is available continuously, or nearly so, is called primary or firm power. The amount of primary power which a given water power plant can produce is measured by the minimum flow of the stream, either natural or as regulated by storage. Power which can be supplied only part of the year, when the flow of the stream is sufficient to produce it, is called secondary or surplus power. Primary power rings the higher price of the two in the market per horse-power year, often about double that of surplus power. The amount of power available at a given location from a natural variable stream may be increased by the use of pondage, or storage, or both.

use of pondage. Storage, on the other hand, is a term usually applied where water which is not otherwise usable,- that is, waters mainly derived from floods or stream flows in excess of the power plant capacities, are stored and held in reserve by natural lakes or artificial reservoirs. Such stored waters are used to make good part of the deficiency in supply of a natural stream during the low water periods. Storage thus increases the average available output of the plant, regardless of the mode of operation, whereas pondage only increases the available output when the demand for power varies from hour to hour during the day.

In the early days of water power development, water power plant capacities were usually made a little greater than the dry weather capacities of the streams supplying them. For example, a common practice was to make a plant capacity equal to the flow of the stream during the third driest month, in other words, the stream would supply the full capacity of the water power plant during nine months of the

10 20 30 40 50 60 70 80 90 100 PERCENTAGE OF TIME A GIVEN POWER IS EXCEEDED FIG. 7.- Water Power Duration Diagram for an Unregulated Stream.

exceeds a given quantity. These time percentages are shown by the scale at the bottom of the diagram. The total area of the diagram o-a-b-g represents the aggregate volume of power available during the year. If a plant having a capacity of say 2,000 horse power is contemplated, then the area of the diagram o-c-d-b-g lying below the line c-d, corresponding to 2,000 horse power, represents the aggregate average yearly volume of power which would be produced by such a plant, while the area a-c-d lying above the plant capacity line represents the volume of power which would be wasted in excess flow or floods. The quantity of firm or primary power which could be produced is represented by the rectangular area. h-b-g-o lying below the lowest point b on the power duration curve, while the area h-c-d-b represents the aggregate amount of secondary

horse power, as indicated by the line j-k, the quantity of firm power would remain the same as before, and only the secondary power would be increased. This in turn would not be increased by so great a proportion as the size and consequent cost of the power plant.

These conditions go to fix and determine the most economical size of water-power installation which can be installed in a given case. Fig. 8 illustrates the effect of a partial regulation of the same stream at the same power site by means of storage. The power duration curve as regulated by storage is represented by the line a-d-m. The average power output is increased by an amount represented by the shaded area d-b-m for a plant of 2,000 horse-power capacity. The firm power is increased to a greater degree, being represented in this instance by the area m-n-o-g.

One of the apparent advantages of storage is that the stored water can be utilized over and over again by successive plants located on the stream below the storage reservoir. There are, however, certain disadvantages attendant on the utilization of storage which reduce its efficiency for power purposes. As a rule where a storage reservoir is located at some distance upstream from a water-power development, it is not feasible to discharge the stored water in such a manner as to regulate the stream during only the working hours, so that if a power plant below the reservoir operates during certain hours of the day only, and does not have sufficient pondage to regulate the flow for a 24-hour period, then the storage which is let out of a reservoir at some point upstream continuously during the dry period, will partially go to waste, the waste in extreme cases, for plants operating 10 hours a day without pondage, being as much as seven-twelfths of the entire volume of stored water released. A storage reservoir, therefore, effects the greatest benefit on streams where power plants are located which operate uniformly and continuously or have extensive pondage in immediate connection with the power developments. The quantity of water reaching a power plant located at some distance downstream from a storage reservoir is the sum of the quantity of water discharged at the reservoir and the natural inflow to the stream channel between the reservoir and the power plant. As the distance from the storage reservoir to the power plant increases, the ratio of the latter to the former also increases, with the result that proceeding downstream from the storage reservoir the degree of regulation afforded thereby progressively de

creases.

The first hydro-electric central station in the United States was installed at Appleton, Wis., in 1882. The use of hydro-electric power for lighting advanced rapidly during the 80's, this advance following quickly the invention of the incandescent lamp by Edison in 1879. While there were some previous experiments carried out, the first large scale successful long distance power transmission was that from Lauffen to Frankfort, completed in 1891, by which 110 horse power was transmitted a distance of 112 miles at 12,000 volts. This provided the initial stimulus for extensive hydro-electric power development. Prior to that date, water power was not much used for the generation of electric