Polytechnic Association. 583 



further east. Its virtue consists in its hardness, and this arises from 

 the large per centage of alumina (the sesquioxide of aluminum) found 

 in it. Xext to the diamond in hardness is sapphire, which is nearly- 

 pure alumina. Corundum contains about ninety per cent of alumina 

 and ten per cent of magnetite, lime and silex, the largest portion 

 of this being magnetite. Emery contains, in place of magnetite,, 

 sesquioxide of iron, the percentage of alumina varying from fifty to 

 eighty, and that of sesquioxide of iron from twenty to fifty. The 

 emery found at Chester, Massachusetts, has the largest proportion of 

 iron. The specimens now shown are from Chester county, Pennsyl- 

 vania, and seem to contain far less iron. All the minerals alluded to 

 are classed under the general term corundum. 



Mr. A. P. II. Stewart exhibited specimens of emery from this mi ne r 

 developed and rendered valuable under the supervision and through 

 the exertions of Captain John Elliot. They are finer than any 

 before reached, and remarkably free from the oxide of iron. lie also 

 exhibited specimens of tourmaline, granite, soapstone, mica and 

 asbestus, from the same region. This emery has been found especi- 

 ally valuable in working upon hardened steel or chilled iron. It has 

 not yet been analyzed. 



Vedder's Self-adjusting Cak-coupler. 



Mr. W. E. Partridge exhibited a model of J. D. Tedder's self-adjust- 

 ing car-coupler, and explained its construction. It requires no change 

 in the framing of the car or truck, and can be applied to any of the cars 

 now in use. There are two springs, seizing and holding the arrow- 

 headed link in coupling the cars, and separated by a lever in uncoup- 

 ling them. This lever may be so shaped as to be operated from any 

 convenient position. When the car is turned at right angles with the 

 link, the springs no longer hold ; so that the car is free, if overturned, 

 to roll down the bank, leaving the remaining cars on the track. 



Mr. C. B. Boyle stated that he had applied tw r enty years ago for a 

 patent for that exact thing. 



Prof. Van der Weyde stated that the Miller patent now in use on 

 the Erie railroad was similar to this, but that this was an improve- 

 ment upon the Miller car-coupler. 



Prof. J. A. Whitney considered the Miller plan the better of the two, 

 there being in that but half the number of parts. The Miller coup- 

 ling prevents the " telescoping " of the cars by avoiding the play of 

 twelve to fourteen inches on each car allowed by the ordinary link. 

 Whether the Yedder coupling will practically uncouple on the over- 



