Faraday to Peter Henry Berthon   4 December 1861

Royal Institution 4 Decr. 1861.

Sir

I beg to report to you my impressions on the view of the Fitzmaurice light at the Trinity House on the 26th ultimo¹[.]

Two lamps were compared one the Fitzmaurice Olefiant gas lamp and the other Wilkin’s oil lamp. Both were of the first order, and the burner of the gas lamp was made to the pattern taken by pressure on paper from the burner of the oil lamp. Both lamps were burnt under exactly the same circumstances of glass, chimney, tube, &c so as to be perfectly comparable as regards the usual optic apparatus[.]

The gas lamp was accompanied by a generating gas furnace of small dimension, containing two retorts, & able to generate gas from resin oil at the stated rate of 25 cubic feet per hour 1 gallon resin oil is said to give 110cf of gas - The gas is lighter than air. There was also a gazometer capable of holding 29 cubic feet of gas. The consumption of the burner is stated to be 40 cubic feet per hour. The furnace went on very steadily in the generation of gas & the lamp was exceedingly regular in its action during the two hour observation[.]

Comparison of the two lights. Both were in good order - the gas flame was rather higher & narrower than the oil flame - it was also whiter in colour. The lights were compared by spherical reflectors - by shadows - by lens images - and appraised to be very nearly equal - at times the gas flame appeared a small degree brightest but the difference of colour was sufficient to account for this. When compared by transmission through many plates of faint reddish coloured glass (to represent fog or obscurity) the oil flame had the advantage;- the advantage was small but constant. Such a gas light therefore will compete with an oil light and as far as the present demonstration goes they are mainly equal. The optical adjustments for an oil flame will approximately serve for the gas flame; but because of the height and shape of the gas flame the beam seaward would with the best adjustment be less concentrated in the vertical direction than that of the oil flame.

I assume that in relation to the different modes of illumination, the Trinity House desires to know of each proposed method, whether dependant on oil, - gas, - the ignition of lime, - or the Electric discharge, what are the circumstances which may render it best fitted for any particular situation and what is the cost and what the liabilities which each involves. Now I do not think that these can be ascertained without a lighthouse trial of some length. The gas light stands well in relation to its claims for a light house trial but it will need the trial just as much as the Electric & lime lights. In this respect the following observations may be useful[.]

The gas light does not appear importantly to exceed the oil light in power when the latter is well sustained (See point 6 in Major FitzMaurice letter 21st Octr²) but certainly if there be plenty of gas prepared before hand the gas flame is more likely to remain unchanged than the oil flame. There is nothing in it which involves the change & care due to the existence of the cotton in the oil lamp. Still that steadiness ought to be confirmed by light house experience[.]

The cost of the gas light is said to be three pence per hour whilst that of the oil light is given at ten pence. This ought to be verified or corrected. I rather think the cost of Manufacture - wages and the interest of outlay is not included - but the whole of this matter should be ascertained in a lighthouse (See point 8 in Major Fitzmaurice letter)[.]

Point 5 in the letter says that one man can prepare the gas;- which should be made in the day & burnt at night. Now a winters night of 16 hours would require 640 cubic feet of gas. This would need for one consumption a gazometer about 10 feet high & 10 feet in diameter in the bell & so much more in depth for the water tank. Point 3 in the letter says a gazometer of 6 feet in diameter (which has little more than a fifth of the capacity of the former) will be sufficient; and a shed 5 feet square will serve for the retorts and manufacture. I doubt these smaller dimensions; but they can only be verified or corrected by lighthouse trial. Point 3 proposes to make and burn gas at the same time. I think that would certainly be unsafe during the first applications of gas to lighthouses.

The furnace & retort at the Trinity house could only manufacture 25 cubic feet per hour:- it would have therefore to work 25 or 26 hours to produce light for one winters night. Larger & efficient furnaces would seem to require more than a shed of 5 feet square; especially if purifiers - and store places for coke and resin oil be included.

It may be expected that gas made at a moderate red heat from resin oil will deposit part of its illuminating portions by keeping especially when subjected to cold; and it is only by winter experience in a lighthouse that the amount of deterioration (if any) due to this cause can properly be ascertained. The effect also of winter cold on the water in the gas holder & gas pipes will have to be watched & ascertained.

The capability of the Extra man and I think also of the keepers to learn to manage the manufacture of the gas can only be ascertained properly at the light house[.]

For these reasons I am obliged to say that I think the trial at the Trinity house does not supersede the necessity of a trial at a lighthouse[.]

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Major Fitzmaurice had also a smaller gas burner, placed, as well as it could be, in the focus of an ordinary 21 inch reflector. The burner was Argand size but it was supplied with a deflector plate which spread out the flame into a cup form. The burner consumed 7 cubic feet of gas per hour and gave much more light than an ordinary Argand oil lamp. This was to be expected. But when both were placed in equal reflectors (Trinity) the oil lamp gave a compressed ray (assumed as 15° of divergence) & the gas lamp - a large diffused ray & when the rays from both were projected onto the screen that from the oil lamp though smaller was more intense at the axis than that from the gas lamp though less intense in the surrounding parts. Such is the relative condition of the oil & gas flames when placed in reflectors³.

I have the honor to be | Sir | Your Very Obedient humble Servant | M. Faraday

P.H. Berthon Esqr | &c &c &c