Page:Popular Science Monthly Volume 32.djvu/880

858 that none such had to beg for work in the middle ages. When a grill was wanted for, say, Westminster Abbey, it was not the local man who had the commission, but a smith from wherever he might be found, who had a designing capacity and a skill of his own, who was fetched and maintained until the task was completed. The smiths of those days were probably not fettered by estimate or bound by time; but, we have a right to suppose, the art-work was produced for art's sake by a genuine artist.

New Kinds of Optical Glass.—Professor Abbé, of Jena, has been experimenting for many years with a view to produce an optical glass which should be free from the defects incidental to all silica glasses. In particular, he sought to produce a higher degree of achromaticity than was hitherto possible, by diminishing the secondary coloring effects inseparable from the ordinary silicate flint and crown glasses, and to produce a greater multiplicity in the gradations of optical glass in respect of the two great constants of the index of refraction and the mean dispersion. In silicate glasses, those two constants increase and decrease together. Cases often arise in which a different relation is desirable. Professor Abbé has produced glasses in which both objects arc fulfilled. He has produced achromatic lenses of a more perfect kind than were ever before obtainable, and has introduced a whole series of new glasses of graduated properties. These glasses are offered freely to the trade without any restriction or patent being allowed to stand in the way of further development.

How the Krakatoa Dust was carried.—Mr. Ralph Abercromby introduces an account of his studies of the relation of the upper wind-currents near the equator, with the diffusion of Krakatoa dust, by showing that, as a rule, there is a continuous successive veering of the equatorial winds as we ascend. Standing with one's back to the surface wind, the upper currents will—north of the equator—come successively more and more from the left with increasing height; south of the equator, the rule is reversed. Nevertheless, some remarkable variances have been observed in the region between the equator and the doldrums. From the I consideration of these exceptional cases the author concludes that when the trades or monsoons meet they do not interlace, as has been suggested by many, but the upper winds combine in a generally easterly current, and probably diverge only slightly pole-ward on either side. The velocity of this current is unknown. Applying this theory to the dust-flow from Krakatoa, as its advance was indicated by the view of green suns and red after-glows, the system of its movement will appear to have been very simple. "The great dust-stream was carried for the first twenty-four hours by the normal easterly upper currents over the southeast trade, at the extraordinary rate of more than one hundred and twenty miles an hour, but hardly extended north of the line. . . . In fact, we may say that the great stream of Krakatoa dust was carried nearly round the world by the usual upper winds of the southeast trade, in which the dust was first ejected at a rate of about one hundred and twenty miles an hour, and that the dust spread very slowly either north or south of the main current." The high velocity of one hundred and twenty miles an hour is certainly more than would have been expected; but, while we have very few observations of the rate of motion of the highest clouds, a number of those that we have give figures approaching this speed. So that the author is able to add: "There would be nothing, then, outrageous in the assumption of a velocity of one hundred and twenty miles an hour for the easterly current over the equator to account for the high speed of the diffusion of Krakatoa dust; and it is also satisfactory to know that the general direction of the flow is in accordance with the most recent researches on the vertical succession of the upper currents near the equator."

The Crater-Lake Chala.—Mr. J. A. Wray last year reached the edge of the water of the crater-lake Chala, on Mount Kilimanjaro, which Mr. Thomson saw and has described in his account of the mountain. The lake is about three miles long by one mile wide, with banks so steep that a descent to the water is impossible, except at one place on the western side. Mr. Wray found the water clear, cool, and perfectly sweet, though