44° 



THE POPULAR SCIENCE MONTHLY. 



tions were begun here in July, 18G9, for the construction of a coffer- 

 dam between high and low water marks ; and in the following Octo- 

 ber the excavation of a shaft, conforming in shape to that of the dam, 

 and 32 feet in depth, was begun. Thence tunnels radiating through 

 the rock, with transverse galleries, 25 feet apart, to connect them, 

 were excavated till thirty-five tunnels and ten galleries were con- 

 structed, having an aggregate length of 7,426 feet. The tunnels were 

 from 17 to 22 feet high and from 9 to 12f feet wide at the shaft, and 

 tapered off in both dimensions as they went out ; and the galleries 

 were from 12 feet high by 9 feet wide down to smaller dimensions. 



Fig. 3.— Hallet's Point. 



The work of excavation was commenced in the latter part of Octo- 

 ber, 18G9, and terminated in June, 1875. Deducting the time lost by 

 suspension of work due to the exhaustion of current appropriations, 

 the actual period consumed in this work was four years and four 

 months. The appropriations were, under the law, devoted to many 

 reefs in the East River and Hell Gate besides the one at Ilallet's 

 Point ; the result being that the work was rarely prosecuted in full 

 force at the latter place. With a more generous grant of money the 

 time consumed until the explosion, which amounted in all to six years 

 and ten months, could have been reduced to four years. 



