Thomas Stevenson to Faraday   22 October 1860

Private<qr>Edinburgh | Oct 22. 1860

My Dear Sir

When at Aberdeen the other day with the Astronomer Royal¹ I recollected of my unfulfilled promise to let you know our average consumption of oil in the Northern Lighthouses. This has been found to be five (5.) hours to one gallon, and whenever they go below that, the keepers are pulled up. In contrasting the whole expenditure of oil at the Northern Lights as compared with the other Boards, this result may not perhaps appear as the Scotch Lights are not lighted from sunset till sunrise. They light and extinguish by a Table in which 40 mins. are allowed for twilight. In this way there is a saving effected of about 1/10th of the whole consumption.

If you should have occasion to try any photometric experiments allow me to suggest a form which after trials of other forms seems very satisfactory. Instead of using the wedge full of coloured liquid, which necessitates the using of different parts of the glass sides, I tried experiments first with a telescope and them with an India rubber bag having glasses placed on each end so that on compressing the bag the glasses were brought nearer each other & the extent of absorbing medium decreased. I ultimately on the suggestion of a friend however recurred to the plan I had thought of before which consisted of a telescopic arrangement placed vertically & having a right angled prism at the bottom for rendering the horizontal rays vertical.

In the accompanying sketch a b c d e f represent the larger tube containing the coloured absorbing medium g.h. the sliding or eye piece tube having a glass at g by which the fluid is prevented from entering the sliding tube. i is the totally reflecting prism. The sliding tube is graduated on the side so that when g is close to the bottom cd the scale on the side is at zero. Of course the reading on the scale increases as g is removed from cd that is as the thickness of the absorbing medium traversed by the rays is increased. I hope the above is intelligible and trusting you will excuse this long letter I remain

My Dear Sir | Yours faithfully | Thomas Stevenson

Professor Faraday | &c &c

Endorsed by Faraday: Consumption of oil 1.6 pints per hour first order