John Wilson Croker to Faraday   7 May 1847¹

West Molesey | Kingston | 7 May 47

My dear Sir

I know not whether the following observations may be worth mentioning to you but I venture to give you the slight trouble of reading them.

During the late winter I observed that three thermometers on different portions of the exterior of this house, seemed to differ more in the marking of the temperature than the difference of position would account for. I therefore brought them altogether to one northern aspect. I then found that the differences were real, but varying; I extract the notations made on certain days, of each of the instruments numbered 1, 2, 3.

On the 23d Feb they all exhibited the same 43 - 43 - 43 and generally it seemed that the higher the temperature the less the differences - the lowest temperatures (we had them as low as 12˚) had passed before I had thought of bringing the instruments into one place; not being, at all, able to account for these differences, which seemed irregular & precarious, I observed, that, by accident, the plates of the instruments were formed of different materials. No 1 had an ivory plate, No 2 a steel one & No 3 one of box wood. It seems to me not improbable that the material of the plate to which it was attached may have had some effect on the mercury in the tube[.]

Of course my first impression was that the instruments were erroneously graduated, but their occasional agreement & varying differences repelled that solution and to establish this, the more perfectly, I have this morning bought them again together, when they all agree exactly - standing at 57 in the hall of this house.

I am | My dear Sir Yours | very faithfully | J.W. Croker

Dr. Faraday