Faraday to Joseph Hume   4 June 1839

Royal Institution, 4 June 1839.

Sir,

On receiving the candles, and your note¹, I hastened the experiments, and now give you the results, begging you to bear in mind that it is only from the average of many experiments that a correct mean can be deduced. I am satisfied that the results I send you are not far from the truth.

A certain amount of light used as a standard in the investigation of the Bude lamp cost, when obtained from an Argand lamp burning in the usual way, 8 1/4d. for 12 hours, the oil being at 6s. 8d. per gallon.

An equal amount of light for the same time, obtained by the Bude lamp, supplied with the same oil, cost 10 1/2d., with all expenses, except that of attendants' wages.

An equal amount of light from the candle (three to the pound), requires 5.32 such candles, and would consume in the 12 hours 1.2335 lb.; which at 1s. 9d. per lb., would cost 26d. nearly.

An equal amount of light from the candle (four to the pound), requires about 6.94 such candles, and would consume in the 12 hours 1.43 lb.; which, at 1s. 9d. per lb., would cost 30d. nearly.

Taking the average of the two candle experiments as the best expression of cost at present, then that light which costs in the Argand lamp - - - - - - - - - - - - - - - - - - 8 1/4d.

Will cost in the Bude lamp - - - - - - - - 10 1/2d.

And will cost in sperm candles - - - - - - 28d.

I remain, &c. | (signed) M. Faraday

Joseph Hume, Esq. M.P. | &c. &c. &c.