PROCEEDINGS OP THE POLYTECHNIC ASSOCIATION. 471 



in forty minutes. A thousand cubic yards have been removed by 

 this dredger in a day ; the average, however, being 350 cubic 

 yards. Roots, bits of wreck, parts of piles, bricks, cannon balls, 

 and a boat grapnel weighing 30 lbs., were pumped up with the 

 sand. One hundred and ninety thousand cubic yards of sand, 

 <fec., have been removed by this machine, working in a sea-way, in 

 two years, at a cost of 60 cents per cubic yard. But the centri- 

 fugal pump is too rapid for this work, pumpir.g only 25 per cent, 

 of sand, and therefore pumps up too large a proportion of water, 

 for the amount of sand raised is only 25 per cent. To prevent a 

 deposit of sand in running water, a velocity of 15 feet per second 

 is necessary. Mr. Johnson then showed that from 85 to 90 per 

 cent, of sand and other earthy matter might be raised by his 

 apparatus. 



The conditions under which bars are formed, are, 1st, the 

 presence of sand, shingle, or other easily moved material ; 2d, 

 water of a depth so limited as to admit of the waves, during 

 storms, acting upon the bottom ; and 3d, such an exposm-e as 

 shall allow of waves being generated of sufficient size to operate 

 on the submerged materials. 



The Chairman. — The Hudson River Railroad company Avould 

 have saved $500,000, if they had known that by the course of 

 the tide a sand bank would have been built up against their 

 bridge ; for they expended that amount in constructing a wall 

 which has proved an unnecessary protection. 



Mr. Johnson showed that sand could be carried through a 

 syphon with a comparatively small proportion of water. There 

 are in most cases few large bodies in the earth to be removed. 

 It is chiefly composed of sand deposited by the tides, and such 

 materials as may be easily pumped in this way. 



Mr. Godwin described the successful application of a similar 

 principle to the emptying of cesspools. 



PATENTS. 



Mr. Churchill inquired what the effect of the publication of an 

 invention by bringing it before this society would be upon the 

 right of the inventor to take out a patent therefor, 



Mr. Stetson. — There is probably no better way to make an 

 inventor's rights known, than such an explanation of an invention 

 as is given here. His right to take out a patent is limited to two 

 years after sale and use, but the right to obtain a patent for an 

 invention made known to the public, never ceases. 



