MECHANICS AND USEFUL ARTS. 25 



THE DIAMOND DRILL. 



The apparatus for boring rocks with diamonds was originally 

 patented in France by Leschot, in 1864, and was rendered prac- 

 tical by Pihet, in 1866. It consists of an iron tube, the end armed 

 with a series of black diamonds of Siberia, which are set in such 

 a way that by turning the tube they excavate an annular groove 

 in the rock, and leave in the centre a solid cylinder which enters 

 the tube, and is easily broken off and extracted when the boring 

 is finished. 15 such machines have already been manufac- 

 tured. The progress is about three-quarters of an inch per 

 minute. The diamonds wear very little ; it is known that this 

 also is the case with the glazier's diamonds, and that the black 

 diamond is a variety much harder than any other. The expense 

 of boring with a machine of this kind is not materially greater 

 than boring in the old way, although more work is turned out ; 

 but the great advantage is, that in the same space where three 

 borers were attached, 8 of these machines may work, requiring 

 not more power to drive them. The expense of excavating 

 tunnels with a single machine of this kind, in hard rock, was 

 found in France to be 40 or 50 francs per cubic metre, which cor- 

 responds to $6 or $8 per cubic yard. 



TANNIN-EXTRACTING APPARATUS. 



At a recent meeting of the Massachusetts Institute of Tech- 

 nolog}', Mr. Langley described a machine in process of construc- 

 tion, on a large scale, at the South Boston Iron Works, for the 

 purpose of extracting tannin from hemlock bark, for the purpose 

 of tanning leather. It is well known that the usual process for 

 tanning leather, so as to obtain a first-rate article, is long and 

 tedious, in a measure owing to the slowness with which the tan- 

 ning principle is obtained from the bark. The bark, in pieces of 

 half an inch to an inch thick, is soaked for 15 minutes in 

 water at about 200 F., and is then fed into a hopper, whence it 

 passes into a machine like a three-rolled sugar or cane mill, pass- 

 ing between the rolls, and coming out torn and compressed to 

 ?uch a degree that most of the tannin-bearing cells are crushed 

 and their contents available ; the crushed product then falls 

 again into a vat of hot water, where it is agitated thoroughly by 

 a wheel ; it is then raised by a series of buckets, somewhat as in 

 a grain-elevator, made of wire netting, to allow the water to drip 

 through ; it is then raised to another hopper, whence it foils into 

 another set of three rolls, coming out finally in flakes or sheets, 

 like coarse paper, almost free from the astringent principle. The 

 extracts thus produced, in a highly concentrated form, and almost 

 saturated, are from four to six times too strong for tanning, and 

 require to be diluted ; they do not ferment, and may therefore be 

 transported easily to parts of the world where tanning materials 

 are difficult to obtain. They possess a beautiful crimson color, 

 and, in the mvst concentrated form, deposit a soft solid subsrance 

 resembling tannin, soluble in cold water. To obviate the black- 



