Faraday to Jacob Herbert   25 April 1844

Royal Institution | April 25 1844

Dear Sir

As I learn that there is no model expected from Mr Neale of the St. Catherines lighthouse in further elucidation of the meaning of his letter & drawing of the 13th instant¹ I will proceed according to your letter² to make such observations on the papers themselves as occur to me[.] In Mr. Neales drawing (returned herewith) Fig 2 represents the top of the central lamp chimney & my ventilating apparatus as it now is - Fig 1 represents the parts as Mr. Neale proposes to arrange them. I have taken the liberty to mark the corresponding parts of the two drawings in red ink for the sake of a clear reference.

At 2 fig 2 is the interval where the chimney of the lamp terminates & my ventilation pipe begins & there the two are separate & apart[.] Now this Mr. Neale if I understand him rightly proposes to close up and so make it what he calls an expansion chamber & I believe this is as to principle the whole of the alteration proposed. Putting this then into plain words it is simply increasing the length of the lamp chimney 1 by joining on to it the first length 3 of my ventilation pipe; this no doubt will increase the draught of the lamp chimney and if the cottons are turned up so high as to give a smokey flame with Fresnels chimney it is very likely that this increase in the length might reduce that smoking. Whether a chimney so lengthened would not burn more oil? whether it would give a better or a worse light? whether the cottons would char sooner? and other matters are questions to be settled by experience. At the same time they are questions which I should imagine had hardly escaped Fresnels attention.

Supposing that the Board may consider the elongation of the chimney as a matter worth trial I would beg to observe that there can be no advantage in a chamber at 2[.] The principles of draught are two [sic] well understood to allow of any such supposition[.] As well might one expect to improve the draught of a common fire place by making a chamber half way up the chimney therefore on lengthening the chimney 1 & 3 had better be one tube of equal diameter or if in two pieces for the sake of moving the upper part out of the way or accommodating glass cylinders of variable heights (which however ought not to occur) then they might be in two pieces sliding one over the other but to have a chamber is not necessary only mystifies the ideas & can be of no advantage[.]

The board will remember that in all I have done to ventilate the lamps I have been especially careful not to interfere with their burning. Without attaining this point I could not have recommended the systems in practice[.] If therefore the lower length of my ventilating tube be in any manner attached to the lamp & made by increase of draught to affect its burning I venture to ask & to hope that it may no longer be considered as part of the ventilating tube[.] In such a case my apparatus will really commence at the space above namely at 4 instead of at 2 and further I would very much wish that no pullies or other connections between it & the lamp chimney were allowed for I cannot then be answerable for or even know what are the variations introduced in different lighthouses & the ventilation may be interfered with & go wrong and yet for no fault of its own.

Mr. Neale says he has tried the experiments first with the dwelling house lamps & with great advantage. This I somewhat doubt & for the following reason. When applying the ventilating system to the reflector lamps of the Tynemouth light the pipes as at first adjusted increased the draught of the glass chimney doing exactly what Mr. Neales elongation of the chimney would do but it was found that this could not be suffered for the wicks charred more quickly & the lights fell in intensity before the usual trimming time came round[.] By adjustment of the draughts & aperture of the pipes I succeeded in having the draught of the lamp unaffected & yet convey away all watery vapour & deleterious gas. If therefore trials are to be made either at the St Catherines light or elsewhere on the effect of elongating the chimney of the lamp, I think it would be better to ascertain & settle that point without the ventilating apparatus (taking it down for the time) & afterwards erecting the latter so that it shall not affect the burning of the lamp[.]

Mr. Neale mentions lastly a fine gauze round the lower part of the burner. This would seem likely to retard the draught & so produce exactly the contrary effect of what is proposed in the early part of the paper & letter[.] But I must acknowledge I cannot understand what the proposition means³[.]

I am Sir | Your Very faithful Servant | M. Faraday

Jacob Herbert Esq | &c &c &c | Trinity House