MECHANICS AND USEFUL ARTS. 77 



bottom, are sufficiently advanced to use rock cars, when the rock 

 will be raised to the top direct from the headings, cars and all. 



The shaft being at grade necessitates, probably, the most deli- 

 cate and responsible professional act an engineer may ever ex- 

 pect to meet, it being necessary to lay down a line less than 27 

 feet in length at the bottom of a dripping dark shaft 1,030 feet 

 deep, so that both ends of the line being produced shall coincide 

 with the terminal points of the tunnel, each being distant over 

 12,000 feet from the centre of the shaft. To increase the initial 

 difficulty, the top of the shaft is on the summit of a rugged moun- 

 tain, from 1,500 to 1,800 feet above the grade of the tunnel at its 

 termini. It is no light responsibility to assume charge of this op- 

 eration. The State of Massachusetts has had manufactured a 

 colossal transit instrument, of the most elaborate and perfect con- 

 struction, costing 3,000 dollars. The most accurately verified 

 lines have been laid down over the mountain, extending long dis- 

 tances beyond in both directions to the tops of neighboring moun- 

 tains. By the accuracy of this instrument and its manipulation, 

 the line of 27 feet (the transverse diameter of the shaft) will be 

 permanently defined, requiring wonderful exactness, and from its 

 extremities the "plummet" alone can reach the bottom of the 

 shaft. These plummets must of necessity be weighty and beauti- 

 fully poised, and will require to be suspended in oil to produce 

 perfect rest and protection from the faintest vibration of the air. 

 The most delicate cords, consistent with strength, must be used 

 to suspend them, and after all is done that science can suggest 

 (being perfect as to theory) any intelligent mind can understand 

 how delicate and fraught with danger is the practical part of the 

 operation to the engineer, and what grave effects the slightest 

 error would produce on so small a base as 27 feet. It is quite 

 possible the motion of the earth may affect the plummets more or 

 less ; but this point has not yet been thoroughly investigated. 



THE HOOSAC TUNNEL. 



The Hoosac Tunnel, it is reported, is now progressing at the 

 rate of 10 feet a day, 4 feet from the west end, and 6 feet from 

 the east end. The central shaft is complete ; its depth to the floor 

 of the tunnel is 1,030 feet. Work at the new headings is already 

 begun. The tunnel has been excavated 11,765 feet at both por- 

 tals, that is, 6,946 feet at east side, and 4,819 feet on the west 

 side. Van Nost. Eng. Mag. 



INTERNATIONAL COMMUNICATION. 



Mr. Bessemer supplies the following description of his inven- 

 tion for improving stearn communication by the construction of 

 a suspended chamber. He says : " The experimental vessel is of 

 only 153 tons' measurement, and, although much too small to at- 

 tain the bust results, is, nevertheless, quite large enough to make 

 the Channel passage, and prove, beyond question, the practica- 

 bility, or otherwise, of the mechanical principle involved. Not 



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