710 Transactions of the American Institute. 



Ponnds 

 Avoirdupois. 



Quartz, tlie pure quartz, if compact, has a specific gravity of 2.6, 

 and a cubic foot weighs 155 



Trap, this useful rock, broken up for paving, has a specific grav- 

 ity of 3.0, and a cubic foot weighs 186 



Vitreo copper, or copper glance, a \vell known copper ore, has a 

 specific gravity of 5.5, and a cubic foot weighs 314 



"Wood tin, or stream tin, has a specific gravity of 7.0, and a cubic 

 foot weighs , 434: 



Zinc — the sulphide or blende has a specific gravity of 4.0, and a 

 cubic foot weighs 248 



Zincite, or red zinc ore, has a specific gravity of 5.5, and a cubic 

 foot weighs 331 



Zinc carbonate, or Smithsonite, has a specific gravity of 4.4, and 

 a cubic foot weighs 268 



Zinc sillicate, or calamine, has a specific gravity of 3.4, and a 

 cubic foot weighs ' 200 



It may be added, for convenience of calculation, that twenty-seven 

 cubic feet are equal to a cubic yard, and that forty cubic feet of arti- 

 cles, nearly equal to the specific gravity of water, are equal to one 

 ton, while one ton of marble is equal to 13.07 cubic feet; granite, 

 13.05 ; common cobble stone, 14.22 ; paving stone, 14.83; sand, 23.05 ; 

 grindstones, 17.00; brick, seventeen cubic feet. 



On a future occasion, these tables will be more extended. 



Many of the weights in the above table are calculated on sixty-two, 

 instead of sixty-two and a half, as the weight of water. The results 

 are theoretically inaccurate, but practically more correct for rough 

 calculations or large masses. 



Underground Railways. 



The regular subject for discussion, viz., underground railways, was 

 then taken up, and Dr. Bradley explained his plan of running a sub- 

 terranean railway from one end of New York city to the other, which 

 plan involves the construction of a tunnel in a straight line and under 

 the blocks of buildings from South Ferry to Harlem Bridge. The 

 doctor proposed to have the work of excavation proceed simultane- 

 ously at short intervals, and believed that with suitable means and 

 appliances the whole could be completed in the short sjDace of six 

 months. Stationary engines would be erected at the intervals to 

 operate the excavating and dirt-can-ying machinery, and after the 

 completion of the way could be enqjloyed to operate elevators for 

 raising and lowering passengers. 



Mr. Gardener introduced the so-called arcade plan in a fluent 



