Definition at the Observatory last night was abominable; therefore no comparisons can be made between the stations. Much obliged for the details of the actinometer. Will call on JH later.

Introduces [Charles?] Cameron. Received angry and excited letter from Charles Babbage. Discusses barometers. Heard Herschels may be travelling to Rio de Janeiro.

Setting a unit of measurement for solar radiation (the 'actine'); proposes to 'take for a unit of solar heat that which if all employed in heating a cubic inch of water exposing a horizontal surface of one square inch, to a vertical Sun during one minute would produce a dilation of one thousandth part of its volume.'