Copy of a letter from Balfour Stewart, Kew Observatory to [Edward Sabine] Stewart received [Sabine’s] letter regarding research the Marquis of [Tweeddale] [contemplates making’. Refers to the manner in which the sun acts on plants. Stewart sees the importance of registering the ‘suns direct heat’. Describes arrangement of the thermometer and refers to the ‘equatorial movement’. Includes a diagram of two curves as an example and how the results will be read. 26 February 1867