Faraday to Peter Henry Berthon   22 October 1857

Royal Institution | 22 October 1857

Sir

Perhaps a few words may be expected from me regarding the second nights observation of the four lights which took place at Hornchurch on the 20th instant. As the place is nearly 10 miles in a direct line from the Wharf, the lights ought to have appeared twice as bright as on the 13th when the distance was 15 miles¹[.] On the contrary they were far inferior because of the misty weather. I did not see them with the naked eye until the latter part of the time; and I learn by enquiry that the weather was so thick at the wharf at first that the persons there did not expect we should see them but it cleared up considerably afterwards[.]

The proportion between the 7 reflectors & the three reflectors appeared to me the same as on the 13th instant, and so did that between the great lens & the narrower Bishops light lenses. To the great lens had been added the fixed light zones above & below but they gave no signs of their presence during the revolution through their eight part of the circle either before or after the appearance of the lens ray.

There was this difference between the observations of this night (the 20th) and the 13th. The 7 reflectors distinctly & considerably surpassed the great lens in the brightness of the light. Also the three reflectors much surpassed the narrow lenses of the Bishops light, the latter indeed were at times hardly visible to the naked eye. This was to me unaccountable until upon close enquiry of Mr Wilkins I learnt that on the first night the Fresnel lamp had burnt too fast & indeed in such a way as would soon have deranged it, whereas on the second night it was kept much lower & in what was considered the proper condition. On the first night they could not enter the revolving apparatus to lower the cottons because of the revolution. On the other hand, the reflector lamps were in much better order on the second night than on the first. They never burn so well on the first night of new cottons as on the second; the 13th. was the first night, & the 20th the second. I believe that these differences are sufficient to explain the differences between the two nights which we observed. I intended to go to the Trinity wharf and see that there was no particular window bar or other obstacle between the lenses & Hornchurch but the weather has been so heavy that there was no chance of seeing Hornchurch from the Wharf. I am assured that nothing was in the way².

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

H.P. [sic] Berthon Esqr | &c &c &c