Faraday to Peter Henry Berthon   23 December 1862

Royal Institution, 23 December 1862.

Sir,

I received, with your last letter¹, five reports made at four different stations, having relation to the Magneto-electric Light now established at Dungeness. The observations are for the month of November, and were made at intervals of three hours simultaneously, i.e., at the same hour and date for all the places.

No. 1 Report is from Dungeness. It reports on the Varne light, distant 11½ nautical miles; the South Foreland high and low lights, distant 20½ miles, and the revolving Grisnez light, distant 23½ miles. The Varne light seems to be generally inferior to the other three. Of the two South Forelands, the upper light is, on the average, the best; but there are occasions when they change their characters. The Grisnez light is reported as generally surpassing the Upper South Foreland light, but not so much or so often as might be expected, considering that it is a revolving light, and that its maximum effect is compared with the constant fixed light of the Fresnel apparatus in the Foreland Lighthouse. The signatures of the keepers are attached every three hours and all in most regular order; nevertheless, I had an impression that some irregularities had occurred which would make this nearly impossible. I should be very sorry to think that the observations were not real, but only a form meant to satisfy the Trinity Board.

There is no report by the engineer of the state and working of the machinery; but from the lanthorn report, and the reports at other stations, I suppose that all has been in accurate and continuous order.

No. 2 Report from the South Foreland high light, which is distant from Dungeness 20½ miles, and from Grisnez 18 miles. The observations in this report make these two lighthouses as almost equal in effect at this place. This seems to me a very important result, if confirmed, for not only is the Dungeness light the farthest off, but it, as a constant light, is compared to the maximum effect of the Grisnez light, which at its maximum is probably eight times as strong as its constant ray would be.

No. 3 Report is from the South Foreland low light; the distances therefore to Dungeness and Grisnez are the same as before. The average results also are so nearly the same, that the remarks made in the last report apply to this also. The observers in this case of the South Foreland reports 2 and 3 are nearly at the same spot, and yet it is curious to see how they differ in very simple observations of the same phenomena. Thus, in relation to the relative superiority of the Grisnez or the Dungeness light column 1st, they often differ from each other in contrary directions, as may be seen by reference to the two reports, at the observations marked as 4th at 6 p.m., 9th at 9 p.m., 9th at 12, 17th at 6 a.m., 20th at 6 p.m., 20th at 9 p.m., 20th at 12, 23d at 12, 24th at 6 a.m. This shows that the two lights must be very nearly equal in their intensity.

The testimony of these two reports is that the Magneto-electric Light has never failed.

There are two reports from the Varne light ship, one signed by John Burton², which I have marked No. 4, and the other by Thomas Davis³ now marked No. 5. The distance to the Dungeness light is 11½ miles, and to the Grisnez light 12½ miles. Out of 106 observations of the two lights as to superiority, 31 and for Grisnez and 75 for the Magneto-electric Light; of the 16 observations made by the reflecting ball, only two give the superiority to the Grisnez light, 14 being in favour of the Dungeness light; so that there seems to be no reason to doubt that the Dungeness light is, at this distance, equal to or superior in effect to the Grisnez light. Then considering the distances are not far from equal; that the Electric Light is constant, and the Grisnez light a revolving light, whose maximum is probably eight times as intense as would be its constant ray, there is every reason to believe that to a sailor’s eye the light of the electric lamp is eight times as strong as that of the lamp in the Grisnez Lighthouse.

The 5th Report seems to be a transcript of the 4th Report, except that it begins on the 1st November instead of the 5th, has no observations by the reflecting ball, but has apparently entered those observations from column 19 of Report 4 into column 18 of Report 5, amongst observations made otherwise; and yet they are all signed by Thomas Davis, as if they were bona fide observations. I do not understand this. Furthermore, in both these reports from the Varne there are figures put into column 3 (state of the weather) which are not directed in the instructions of page 1 of the reports, and are nowhere explained.

The observations by the ball (column 19) in Burton’s report are far too few; I expected them to be made at every third hour during the night⁴.

I am, &c., | (signed) M. Faraday.

P.H. Berthon, Esq.