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at a total cost of £5,000,000 to irrigate an additional 100,000 acres in the Transvaal. Unfortunately, it is on far too ambitious a scale for the present requirements of the two Colonies, and it cannot be commenced on a smaller one and be subsequently enlarged. The time may come, however, when it will be wanted, and it is as well that it should not be lost sight of.
Schemes of the magnitude described above may have the advantage of rendering the climate in the neighbourhood more humid, and, if this proves to be the case, their benefit will extend beyond their own immediate limits.
13. INTER-COLONIAL CO-OPERATION.
Irrigation schemes of all sizes have now been discussed broadly, and chiefly from the engineering point of view. It has been pointed out that each class of scheme has its own special utility, and that, if any scheme fulfils the main requirements of feasibility, suitability, desirability, and moderate financial success, it can be undertaken with confidence. It is, perhaps, needless to add that each project should be carefully and fully investigated before it is started, so as to avoid the commission of costly mistakes. It now remains to make a few general observations on the main subject of this paper.
Although the Colonies are at present only in their preliminary stage of development, difficulties have arisen in regard to water questions connected with the Vaal, owing to the deficiency of its supply at the end of the fair season. During each of the last three years its flow at Christiana has practically ceased for two or three months, and is likely to fail this year for a considerably longer period. The restriction of irrigation from its upper tributaries has even been suggested, so as then to secure supply to the lower riparian towns in Cape Colony, including Kimberley. It is hardly necessary to point out that, even if this were practicable, it would be a retrograde step to take, and it would introduce inter-colonial friction instead of co-operation. It is, however, not practicable, as the laws of each Colony run only within its boundaries, and the upper Colony would certainly never consent to being deprived of its natural resources for the benefit of the lower Colony. Considering the case of two Colonies separated by a river, the best way, theoretically, is to divide its flow between them in proportion to their contributions to its discharge. Such discharge depends upon the extent of the catchment area, the nature of the country, and the intensity and amount of the rainfall. To determine the discharge to which each Colony has a fair claim would take many years of observation, and, even when determined from the results ascertained, the proportion arrived at might, with the variation of rainfall in a subsequent season, then prove an unfair one. Owing to the great fluctuation in the flow of even these large rivers, to depend upon them in their natural condition will frequently lead to difficulty. The obvious remedy is to reduce these fluctuations artificially by means of storage weirs and dams, whereby a definite amount of supply can be
obtained in all years, and can be divided in acc ance with settled arrangements, effected jointly by the Colonies concerned. Thus all chance of inter-colonial friction will be prevented by means of intercolonial co-operation, which will establish a community of interests that must inevitably form a bond of union. An excellent instance of the benefits of such co-operation is the appointment of the InterColonial Irrigation Commission, upon which are representatives of the Transvaal and Orange River Colony. This Commission is enquiring into the best way in which the existing irrigation law can be amended so as to meet the altered conditions which now exist. Without
law suitable to those conditions, the proper development of irrigation will be impossible.
The tendency in new countries (and one which, unless it is extirpated at the outset, may be intensified as time goes on) is to consider first, individual, rather than collective requirements, and to look at everything from a narrow rather than from a broad point of view. Fortunately, South Africa has already furnished several examples of the advantage of co-operation. In State affairs there are the Customs Union, and the joint management of railways in the Transvaal and Orange River Colonies, which, it is hoped, will be extended to Cape Colony. In commercial affairs there are the gold fields of the Rand, which, under the most adverse natural conditions, produce the largest output in the world, and the Kimberley Diamond Mines, which, from a congeries of petty individual effort, have developed into the most productive combination known. By this co-operation scientific development has enormously advanced, and, by a similar one, it is probable that irrigation engineering will equally benefit for the good of agriculture all over the Colonies. Agriculture is the oldest and most permanent industry in the world, and is practised by the large bulk of its inhabitants. Anything that tends to its development and renders it more certain will benefit the whole population, and nothing can ensure this better in South Africa than the construction of large irrigation schemes on well-considered and sound lines.
STATEMENT OF IMPORTS OF AGRICULTURAL PRODUCTS INTO
For the Year ending 30th June, 1905.
(Report of Agricultural Department, 1904-05, page 44.)
1,689 Kaffir Corn
5,697,807 17,464 Lucerne and Fodder
36,493,257 81,385 Manna...
88,645,632 194,324 Oathay
Note.--In addition to these, considerable quantities of butter, milk, cheese,
margarine, eggs, bacon and ham were imported. ---W.L.S.).
By H. BOHLE, M.V.D.E., M.I.E.E., PROFESSOR OF ELECTRO
TECHNICS AT SOUTH AFRICAN COLLEGE.
When electrical energy has to be transmitted over great distances or distributed over large areas, high pressure alternating current is used in order to limit the expenditure in copper.
If the energy is used solely for power purposes, a low frequency current is preferable, since the construction of motors and rotary converters is more rational for periodicities below 40 than for those above it. For pure lighting networks, however, the frequency should be 50 or more, as arc lamps do not burn satisfactorily on a lower frequency.
For mixed lighting and power plants, we usually effect a compromise, i.e., we choose a frequency of about 50, which is ideal neither for lighting nor for power.
As regards the number of phases, it is well known that polyphase rotaries and motors are superior in starting and running qualities than single phase machines, and less expensive to manufacture. The pressure regulation, however, is simpler for single phase than for polyphase currents. One is further justified in allowing a greater pressure drop in power than in lighting circuits, the drop being much less noticeable in the former case than in the latter. Consequently, if we have a mixed lighting and power network, the allowable drop is soon reached, and the expenditure in copper is larger than for a pure power plant of the same total magnitude.
It is further necessary, on account of the low voltage of the present day incandescent lamp, to employ a pressure below 300 volts for lighting circuits, while it is allowed to run motors from circuits up to 500 volts.
It is therefore obvious that the quantity of copper required for a mixed circuit is greater than for a pure power network of the same magnitude.
The disadvantages of a mixed plant are overcome by having separate plants for lighting and for power purposes; for instance, by employing two phase or three phase currents of 25 periods for power circuits, and single phase current of 60 or more for lighting networks. This, however, increases the capital outlay, and complicates, the switching arrangements.
A more rational solution of this intricate problem is found in a system which transmits at the same time currents of different frequencies and phases through the same network.
Suppose we have two sine currents of the frequencies f, and fa, produced by sine E.M.F.s of similar periodicities, then the current of frequency f, performs work with the E.M.F. of the same frequency,
but not with the E.M.F. of frequency f2. If one current, say ii, varies, its E.M.F. will also change, but not the other E.M.F., as long as i, remains constant, because the two currents are absolutely
independent. The total power absorbed by the circuit is equal to the sum of the powers of the two currents, i.e.,
P=Ejlı cos 01 + E,12 cos 02,
and the copper loss caused by the currents in the resistance R is equal to (I,' +1.) R.
The author believes that the first polycyclic system was due to Dr. F. Bedell, the connections of a three phase single phase system, as invented by him, being shown in Fig. I. Here G, and Gi represent three phase and single phase generators respectively, T, and T, three and single phase transf hers, M, a three phase motor, and L a lighting circuit.
Suppose now that the currents of the different phases of G, are equal and follow sine laws, then points o, and o: will be of the same potential, so that a single phase current may be introduced in these points, passing through the varies circuits as if no other currents were present. The direction of the superposed current only is indicated by the arrows.
A single phase current may be superposed not only upon three phase, but also upon two phase or another single phase currents. The latter is then considered a two phase current with phases at 180 degrees, see Fig. 2.
Bedell's system has the disadvantage that the superposed currents are carried by the primary system only, and that, since the circuits of the superposed current are highly inductive, the pressure drop is enormous.
The latter disadvantage may easily be obviated by winding the circuits noninductively for the superposed current without interfering with the induction of the original currents. Consider the choking coil as shown in Fig. 3, and assume that the superposed current enters by b, and b2, leaving at e, while the original current enters at b, and leaves, at ba; it is obvious that the inductive action of the superposed current is nil, since the flux caused by one coil is neutralised by that set up by the other on the same limb, whereas the inductive actions of the original currents are added. Such bifilar windings may be given to all kinds of polyphase transformers and generators, as illustrated in Figs. 4 and 5. The former illustrates a three phase transformer, the latter a three phase generator or motor.
Arnold-Bragstad-la Cour, the inventors of this system, have further shown that superposed currents may not only be conducted by means of transformers, but also be induced by them. The principle of this new method consists of the simultaneous transformation and supply of alternating currents of different phases and frequencies into the mains of a distributing network by employing transformers with two kinds of primary and one secondary windings, and by taking off these currents by means of one primary and two secondaries.