Frederick Madan to Faraday   6 September 1842

Trinity House | Septr 6th / 42

My dear Sir

It seems evident that the refraction is included in the Tables which I have consulted, indeed, in an example given by Kerigan¹, one tenth is clearly deducted for terrestrial refraction at an altitude of 290 feet²: & Lieut Raper³, whose tables I have since looked into, gives 25' as the true depression for 550 feet⁴. It would probably however be best to take it, as you recommend, by observation, altho' I for one should be well satisfied to decide at once upon 1˚ as the best depression for the apparatus.

In regard to my question about altering the position of the Lenses, you have not understood me right. I meant of course that the Lamp should be raised, as required, & in addition I asked if it would not also be desirable to fix the Lenses a little out of the perpendicular to trim them to the Light. I doubt not you are correct, that it would make little or no difference practically, as the alteration in the height of the Lamp is so small, but this I apprehend cannot be proved to demonstration, & Captn Fitzroy⁵ agrees with me that it would be more satisfactory to have the apparatus quite perfect, in the event of it's being visited & examined scientifically at any future day; & Mr. Wilkins seemed to say that in putting the Lenses into the frame, he could make a slight alteration without difficulty, & independent of the principal framework.

Capt Drew promises to bring the subject before the Deputy Master⁶ the first opportunity.

believe me | Yours very sincerely | Fredk Madan