often a benefit, which appears to us injurious only because we do not understand it fully, every disturbing cause tends to produce some apparent disorder, such as storms, hurricanes, &c., some indeed so terrific as to destroy ships and houses, but what would be the result if the atmosphere were never disturbed from any cause? Why the lower stratum would become so loaded with impurities that it would be unfit to breathe, miasms and noxious gases would for ever remain a curse to the races of men and animals who might be doomed to inhabit such regions, but the very regions where these miasms are most likely to form are those about the tropics, and here it is that the greatest storms occur to remove them. Even the great deserts, which appear so useless to man, and which are uninhabitable to a great extent, have their office, and an important one too; they are to the earth what ventilators are to buildings, drawing the cold air from the poles to cool the regions that are too hot, and sending a current of heated air through the upper regions of the atmosphere (where it can do no injury to anything) to warm the colder parts of the earth, another instance of the wonderful care and goodness on the part of the Creator. The various and beautiful colours of the clouds, particularly at the rising and setting of the sun, are caused by refraction separating the white light into its primitive constituents, blue, red, and yellow light and their combinations, purple, orange, green, &c., and the more obliquely the rays impinge upon the earth the greater will be this refraction, this accounts for colours seldom appearing in the clouds at midday. But of all the beautiful effects of the refraction of light, the Rainbow (fig. 9) is the most glorious, it has been celebrated in all ages for its transient beauty. It is only seen when rain is falling in front of a brightly illuminated cloud, the sun being behind the spectator; it is a reflection of the sun by the cloud transmitted through millions of drops of rain, each of which acts as a prism, and produces rings of colour; for each of the rays of light (red, blue, and yellow) are refracted in unequal degrees, and therefore take separate places, forming the rings of colour seen in the rainbow. [Illustration: FIG. 9.]