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The difficulty of securing in England a sufficient number of consecutive cloudless days to render it possible to determine with any degree of accuracy the relation existing between the sun’s altitude and the chemical intensity of total daylight, induced us to undertake a series of measurements on the west coast of Portugal, where during the months of July and August the sky is generally cloudless. The method of measurement employed was that described by one of us in previous communications to the Royal Society, founded upon an exact estimation of the tint which standard sensitive paper assumes when exposed for a given time to the action of daylight. The observations, the results of which are given in the following communication, were made in the autumn of 1867, through the kindness of Thomas Creswell, Esq., at the Quinta do Estero Furado, situated on the flat table-land on the southern side of the Tagus, about 8½ miles to the south-east of Lisbon, lat. 38° 40' N. and long. 9° W. The sensitive paper was exposed in the plane of the horizon, the instrument being placed upon a carefully levelled stand at the height of 4 feet 5 inches above the level of the ground in a sandy field having a clear horizon, the most considerable object in the neighbourhood being a house distant 130 paces to the west, the roof of which subtended an angle of 7° with the plane of the paper.

XVII. On the relation between the sun's altitude and the chemical intensity of total daylight in a cloudless sky