Page:Popular Science Monthly Volume 7.djvu/35

Rh to produce photographic effects which in New York require only a minute; and the further statement of travelers, engaged in copying the antiquities of Yucatan, that they frequently have been obliged to abandon the use of the camera, and take to their sketch-books, have led to some investigations, similar to those at Kew, for determining the intensity of the chemically-active rays in the tropics. Prof. Thorpe experimented at Pará, situated nearly under the equator, in the northern province of the Brazils, and lying on a branch of the Amazon. Of the results. Prof, Roscoe remarks: "Owing to the rainy sear son having commenced when the experiments were made, the changes in the chemical intensity, as observed from hour to hour, and even from minute to minute, are very sudden and remarkable; this is well shown by the zigzag lines of Figs. 5 and 6; and these, compared with the dotted lines below, indicating the corresponding action on the same



day at Kew, show the enormous variation in chemical intensity which occurs under a tropical sun in the rainy season. Regularly every afternoon, and frequently at other hours of the day, enormous thunder-clouds obscure the sky, and, discharging their contents in the form of deluging rain, reduce the chemical action nearly to zero. The storm quickly passes over, and the chemical intensity rapidly rises to Its normal value. By comparing the curves for Pará and Kew on the same days, we obtain some idea of the energy of chemical action at the tropics, and it is at once evident that the alleged failure of the photographer cannot at any rate be ascribed to a diminution in the sun's chemical intensity, which, in the month of April, 1866, was nearly seven times as great at Pará as at Kew."