858 



THE POPULAR SCIENCE MONTHLY. 



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 

 Abbe, of Jena, has been experimenting for 

 many years with a view to produce an op- 

 tical 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 col- 

 oring effects inseparable from the ordinary 

 silicate flint and crown glasses, and to pro- 

 duce 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 to- 

 gether. Cases often arise in which a differ- 

 ent relation is desirable. Professor Abbe 

 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 pat- 

 ent being allowed to stand in the way of 

 further development. 



Dow the Krakatoa Dnst was carried. — 



Mr. Ralph Abcrcromby introduces an ac- 

 count 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 succes- 

 sive 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 rcRion between 



the equator and the doldrums. From the 

 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 cur- 

 rent, 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 du^t-stream was carried 

 for the first twenty-four hours by the nor- 

 mal easterly upper currents over the south- 

 east 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 south- 

 east 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 cither 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 nun:ber 

 of those that we have give figures approach- 

 ing 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 Kiliman- 

 jaro, which Mr. Thomson saw and has de- 

 scribed 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 



