PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 509 



MODES OF BLOCKADING. 



On motion of Mr. Dibben, the subject of blockading Avas post- 

 poned until the next meeting, and extended, so as to include 

 " Modes of Obstructing the Navigation of Rivers and Harbors." 



Adjourned. 



American Institute, Polytechnic Association, 



February 13, 1862. 

 Mr. S. D. Tillman in the chair. 



Mr. Abraham Brower, No. 4 Reade street, stated that he had 

 patented a water proof composition for leather articles, but 

 declined to state what its ingredients are. 

 It was referred to the Chemical Section. 



MODES OF BLOCKADING. 



Mr. Fisher read a paper upon " modes of obstructing the navi- 

 gation of rivers and harbors." 



After giving a history of the recorded instances of the obstruc- 

 tion -of harbors apd the interruption of navigation by artificial 

 barriers, and discussing the principles of international law appli- 

 cable to the subject, he described the method adopted at the 

 entrance of Charleston harbor, and then proceeded as follows : 



On engineering principles, I object to this method of Vdocka- 

 ding. Its first cost is greater than that of a more elHcient method, 

 which has recently been exhibited to this club, by Mr. Johnson, 

 and the removal of stones and hulks will be far more costly than 

 the removal of the bars formed by the means Mr. Johnson 

 proposes. 



As I understood the plan, Mr. Johnson proposes to lift sand or 

 mud, through a tube, into a large chamber or vessel, by means of 

 an air pump, or water pump, operating to form a partial vacuum 

 in that chamber. When the chamber is filled with sand, the 

 vessel is to go to the place where it is to be dumped, to dump it, 

 and then to return for another cargo, and so on. 



While he was raising the sand, in the apparatus he exhibited, 

 it occurred to me that a centrifugal pump, driven at the speed he 

 stated it to be necessary, might raise the sand in a constant stream, 

 and force it through hose, for a sufficient distance to form 

 embankments under water. He stated "that, in some practice in 

 dredging by pumps, about three-fourths water and one-fourth 

 sand had been pumped ; but this excess of water was due to too 



