A variant of the group of cases given in Figs. 10 is shown in Figs. 11, in which

Q is made to vary with T,, as it would do in an actual engine owing to the smaller weight of charge that would be taken in with a higher suction

temperature. It will be noticed that a considerable alteration is thus produced in the diagrams.

It need hardly be pointed out that these theoretical diagrams are in a sense

purely imaginary, owing to the high temperatures and pressures shown. Figs. 12 have therefore been drawn giving the temperature-entropy and pressure-volume diagrams for an ideal engine and for a probable actual engine, so as to get some

[THE INST. C.E. VOL. CLXII.]

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Proceedings.]

idea of the relations that would exist in actual engines in which the variables were altered as above. It is to be noticed that the higher the temperature the greater will be the loss to the jacket, and, further, the higher the pressure

the greater the leakage past the piston; thus there is a tendency in actual engines to tone down the differences that appear in the ideal engines as given above.

Meeting.]

EXTRA MEETING.

10 April, 1905.

ALEXANDER BLACKIE WILLIAM KENNEDY, LL.D., F. R.S., Vice-President, in the Chair.

THE "JAMES FORREST" LECTURE, 1905.

THE CHAIRMAN remarked that in the unavoidable absence of Sir Guilford Molesworth, the President, who had fully intended to be present, he had to take his place. Colonel Crompton did not require any introduction to those present. The subject of the lecture was "Unsolved Problems in Electrical Engineering "; and no one was better qualified to speak on such a subject than one who had already solved so many electrical problems as Colonel Crompton had.

The following lecture was then delivered:

"Unsolved Problems in Electrical Engineering."

By Colonel ROOKES EVELYN BELL CROMPTON, C.B., M. Inst. C.E.

The electrical problems which form my subject divide themselves into two groups: those set for us by Nature herself, which chiefly concern the scientific investigator; and those which have presented themselves to us engineers since we began to use electrical energy for man's service. The engineer is only concerned with Nature's problems so far as their investigation comes into his work when he is called upon to provide means to protect our works from damage due to her display of electrical forces.

The problems of lightning-discharges have from the very first interested us, and in these later days of large power-schemes the importance of protecting our works against lightning-stroke has become increasingly evident. For a long time we thought that the damage caused by lightning was confined to systems of overhead conductors, as its effect on these was at once evident, in some cases by

complete destruction of parts of our circuits, in others by the breakdown of our insulation, but during our recent developments of large power-schemes, employing systems of conductors to carry the energy into districts where the use of overhead conductors is inadmissible, the influence of lightning-discharges, or of earth-strokes connected with them, on our underground systems, has been undoubtedly the cause of many failures and interruptions to service, the investigation of which forms a problem of the first magnitude.

The problem has been insufficiently investigated, but it appears probable that many of the mysterious perforations of the insulation. of our underground cables are commenced by static charges due to condenser-effects produced on a gigantic scale in the system.

I have been slowly gathering evidence during the last few years, and I find that most of the engineers who have studied this question are in agreement with me, that our present methods of detecting or guarding against the effects of frequent and hitherto unnoticed static discharges are altogether inadequate to protect our works, and that we have here presented a question of extreme difficulty and importance. No doubt in our temperate climate these effects are not so often manifested, nor are they so important, as in the more extreme climates of America, or in the parts of India where I have had personal experience of them. It is probable that they do occur in this country, and that damage to our underground systems of mains, hitherto unexplained, is due in many cases to these atmospheric or earth discharges; but only those who, like myself, have stood in a power-house in the Himalayas and witnessed the almost continual series of discharges which follow one another so rapidly that they resemble a continuous cannonade, are able fully to realize how liable our works are to become a link in the distribution of Nature's gigantic electrical display of power.

During a recent conversation with Professor Elihu Thomson in America we discussed this question, and he reminded me of the undoubted fact that lightning-strokes as they pass through the air very frequently follow one another over nearly, if not identically, the same air-path. He seems to think-but this requires confirmation that the dielectric strength of the air at this point is reduced or partly broken down by the passage of a powerful lightningstroke, which facilitates the passage of subsequent strokes at the same point. Are we to conclude that corresponding conditions of weakened path are set up below the earth's surface, and consequently affect us by affording paths for disruptive discharge through our underground mains?

Closely allied with the investigation of these interesting pheno