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XVII. On the Relation between the Suns Altitude and the Chemical Intensity of Total 
Daylight in a Cloudless Sky. By Henry E. Eoscoe, F.R.S., Professor of Chemistry 
in Owens College , Manchester , and T. E. Thorpe, Ph.D., Professor of Chemistry in 
Anderson's University , Glasgoiv. 
Keceived March 3, — Read March 31, 1870. 
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 Koyal 
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 8^ 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. 
All the experiments were made in the following order : — 
1. The chemical action of total daylight was observed in the ordinary manner. 
2. The chemical action of the diffused daylight was then observed by throwing on 
to the exposed portion of the sensitive paper the shadow of a small blackened 
brass ball placed at such a distance that its apparent diameter seen from the 
position of the paper was slightly larger than that of the sun’s disk. 
3. The chemical action of total daylight was again determined. 
4. That of the diffused daylight was a second time ascertained. 
The means of Observations 1 and 3 and of 2 and 4 were then taken. The sun’s alti- 
tude was determined immediately before and immediately after the foregoing observa- 
tions of chemical intensity, the altitude at the time of observation being ascertained by 
interpolation. A box-sextant made by Worthington, of London, was employed with an 
*' Bakerian Lecture, Philosophical Transactions, 1865, Part II. p. 605. 
2 T 
MDCCCLXX. 
