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 brought, but as it had been carried to a great height, it was in a position to be conveyed to almost any distance by a violent wind, had such occurred at the time.

The numerous cases of sand and volcanic dust being carried enormous distances through the atmosphere sufficiently prove the importance of wind as a carrier of solid matter, but unfortunately the matter collected has not been hitherto examined with a view to determine the maximum size and weight of the particles. A few facts, however, have been kindly furnished me by Professor Judd, F.R.S. Some dust which fell at Genoa on 15th October 1885, and was believed to have been brought from the African desert, consisted of quartz, hornblende, and other minerals, and contained particles having a diameter of 1 / 500 inch, each weighing 1 / 200,000 grain. This dust had probably travelled over 600 miles. In the dust from Krakatoa, which fell at Batavia, about 100 miles distant, during the great eruption, there are many solid particles even larger than those mentioned above. Some of this dust was given me by Professor Judd, and I found in it several ovoid particles of a much larger size, being 1 / 50 inch long, and 1 / 70 wide and deep. The dust from the same eruption, which fell on board the ship Arabella, 970 miles from the volcano, also contained solid particles 1 / 500 inch diameter. Mr. John Murray of the Challenger Expedition writes to me that he finds in the deep sea deposits 500 and even 700 miles west of the coast of Africa, rounded particles of quartz, having a diameter of 1 / 250 inch, and similar particles are found at equally great distances from the south-west coasts of Australia; and he considers these to be atmospheric dust carried to that distance by the wind. Taking the sp. gr. of quartz at 2.6, these particles would weigh about 1 / 25,000 grain each. These interesting facts can, however, by no means be taken as indicating the extreme limits of the power of wind in carrying solid particles. During the Krakatoa eruption no gale of special violence occurred, and the region is one of comparative calms. The grains of quartz found by Mr. Murray more nearly indicate the limit, but the very small portions of matter brought up by the dredge, as