With a view to the introduction of the instrument into meteorology, and as part of the routine work of an observatory, he caused a regular series of measurements to be made during two years at the Kew Observatory, under the direction of Dr. Balfour Stewart; and in order to gain further knowledge of the variation in the chemical action of light in different areas of the earth’s surface, he sent the writer in 1866 to Pará, on the river Amazon, 1° 28´ south of the Equator. The selection of this particular place arose from the circumstance that his cousin, now the Right Hon. Charles Booth, was proceeding to the Brazils in connection with the establishment of a new line of steamers, and it was arranged that the writer should accompany him, in order that he might make photometric observations _en route_, going and returning, and at the same time make a series of determinations of the amount of carbon dioxide in sea air, night and day, with a view of testing the truth of an allegation by a French chemist, that it was subject to a diurnal variation, depending upon the intensity of sunlight. The special form of apparatus arranged for making photometric observations at sea proved to be ill-adapted to the purpose. But even if its performance had been good, the conditions under which it had to be employed were incapable of furnishing valid results. Accordingly the writer elected to remain at Pará in order to obtain the required observations at that place, until such time as he could return by the succeeding steamer. He was thus enabled to make a much more extensive set of observations than was originally contemplated, and under conditions which ensured trustworthy measurements. The following letter, dated May 12, 1866, and received at Pará, refers to these matters. The allusion to Agassiz arose from the circumstance that the great naturalist was at the time engaged in work on the Amazons and its tributaries under the auspices of the Emperor Dom Pedro. Roscoe himself was busily engaged with his vanadium researches. Although I was disappointed to find that the _Augustine_ returned without you, yet I think that you acted quite rightly in staying until the _Jerome_ returns, as you could not get any results whilst on board. I only hope that your health will have been good, and that you will have enjoyed your stay at Pará, and that when you return in August we may have to work out plenty of interesting results. I send per the _Jerome_ a second bottle of silver solution, and some more salted paper, as you may possibly be out of both. I also enclose two or three fixed strips, but _not calibrated_. These, in case you are out, may be used, _carefully preserved_ and calibrated, on your return. They must be carefully marked before using and notice taken in the book of the marks on each strip when employed. …