142 



SCIENCE. 



[Vol. XIV. No. 343 



operate eight motor cars of thirty horse-power each, and to light 

 four horse-stables, three car-barns, and the generating station, — 

 about five hundred lights of sixteen-candle power each. 



The road as at present equipped is two and eighty-five one- 

 hundredlhs miles long, and has three grades, each about sixteen 

 hundred feet long with a three per cent rise. On the day of trial 

 a motor car with a trailing car attached, loaded very heavily, 

 made the distance in twelve minutes, and while on the grades the 

 speed obtained was as high with the car added as that of the mo- 

 tor car by itself, showing conclusively that the Thomson-Houston 

 motors are of sufficient capacity to do in the most satisfactory 

 manner the work cut out for them. There is no unpleasant jerk 

 or jar in starting the car, such as is found in many systems, and no 

 scientific or mechanical knowledge is necessary to handle the car, 

 which is controlled with ease. 



The road when finished will be nearly six miles in length, mak- 

 ing a round trip of twelve miles ; and this, if proven successful, 

 will be but a start in the rapid-transit line by the Consolidated 

 Street Railroad Company. They have under way the plans for a 

 one thousand horse-power plant for the hills, and the present plant 

 will be enlarged to accommodate at least two more down- town 

 lines. They also contemplate the equipment of from fifty to sev- 

 enty-five cars, and expect to have the whole completed and in run- 

 ning order by the first of next year. 



The recognition by this company of the merits of the Thomson- 

 Houston electric railroad system is a very strong point in favor of 

 that company; and as they gave practical demonstrations of their 

 ability to fulfil their promises, they undoubtedly merit the honor 

 thus paid them. 



THE PCETSCH FREEZING PROCESS IN MINING 

 OPERATIONS. 



A BRIEF description of the freezing process devised by Herman 

 Poetsch for sinking shafts in quicksands and other difficult ground 

 was given in these columns in April last. The process has been 

 successfully applied in sinking a shaft for the Chapin Mining Com- 

 pany at the Iron Mountain mines in Michigan. In this case, so 

 thoroughly and effectively was the freezing done that, although the 

 shaft was finished some two months ago, the earth surrounding it 

 is still frozen solid in places. 



The following description of the difficulties overcome and the 

 methods employed in sinking the shaft mentioned is furnished us 

 by the Poetsch-Sooysmith Freezing Company of this city, who con- 

 trol the patents covering the process in this country. 



A shaft fifteen and a half by sixteen and a half feet was to be 

 excavated through quicksand to a ledge about a hundred feet be- 

 low the surface. The mining company put the freezing pipes into 

 the ground three feet apart, in a circle twenty-nine feet in diameter, 

 and, with the exception of two of the pipes, down to the ledge. 

 This proved to be a difficult task on account of the many bowlders 

 encountered. A ten-inch casing pipe with flush joints was first 

 drilled down by various means, a drill being worked within the 

 pipe when necessary and the material removed by jetting or by a 

 sand pump. The casing pipe being once down to the ledge, a 

 freezing pipe was placed inside, and the outer casing pipe drawn 

 up and used for the next pipe. The freezing pipes left in the 

 ground were eight inches in diameter, the lower ends being closed. 

 Inside of these eight-inch pipes were placed pipes one and a half 

 inches in diameter, open at the bottom. These inner pipes, as well 

 as the outer pipes, were connected together at the top of the ground, 

 as shown in the pipes at the left of the illustration, forming a com- 

 plete circuit, through which a cold brine was circulated. 



The brine used was a solution containing about twenty-five per 

 cent of calcium chloride, which has a very low freezing point. The 

 brine was cooled with an ice machine, having a refrigerating capac- 

 ity .of fifty tons of ice per day. The ammonia was compressed to 

 about 135 pounds per square inch, and cooled by passing through 

 coils immersed in water kept cold by pumping from a brook. Then 

 the ammonia was allowed to expand through coils immersed in the 

 brine and finally returned to the compressor. 



The temperature of the expanded ammonia was such as to cool 

 the brine to a few degrees below zero, Fahrenheit. This brine, 



being circulated through the ground pipes, was raised in tempera- 

 ture about 2° F. After forty days' freezing, an ice wall ten feet 

 thick was formed around the shaft. The excavation, commenced 

 soon after starting the ice machine, had in the meantime reached a 

 depth of forty feet. Thirty days more sufficed to reach the ledge. 

 The shaft was, for convenience, curbed as the excavation proceeded. 

 This was, however, not necessary, as the walls would have stood 

 vertically throughout the whole depth very well. The temperature 

 of the air within the shaft was generally below the freezing point, 

 and there was no indication of the exposed material thawing. The 

 curbing was made -of horizontal sets of timbers, sixteen inches 

 square, placed two feet apart, with four-inch vertical plank 

 behind the timbers. The cross walls were put in place after- 

 wards. 



The timbering was supported from one set to another by bolts 

 placed near the corners of the shaft, the whole system being sus- 

 pended from cross timbers at the surface of the ground. The un- 

 frozen area within the shaft grew less as the actual running time 

 of the freezing machine increased. By the time a stratum of 

 bowlders was encountered, the frozen area reached nearly across 

 the shaft ; but when quicksand containing a large percentage of 

 water was passed through, the unfrozen area was greater. The 

 reason of this is readily understood when it is remembered that the 

 specific heat of water is about five times as great as that of any of 

 the other materials, and therefore the strata containing most water 

 would require more cold and would be longer in freezing. 



The hardness and appearance of the fractures of the frozen 

 quicksand approached those of sandstone. Granite bowlders em- 

 bedded in it showed a decided tendency to fracture across rather 

 than break loose. The tensile strength of the frozen ground, as 

 determined by a cement-testing machine, was equal to that of the 

 best neat Portland cement, and varied from 350 to 450 pounds per 

 square inch, and its strength against crushing, as determined from 

 inch square cubes, was 850 pounds per square inch. This furnishes 

 data from which the strength of the surrounding frozen wall may 

 be computed as an arch. An ice wall ten feet thick will be found 

 sufficiently strong for any case likely to occur. Near the bottom 

 the freezing extended within the circle solidly ten feet from the 

 pipes. It is not known how far it extended outside, as no borings 

 could be made through it. A test pit was sunk outside the shaft 

 as far as the water would permit (some fifteen feet), and from this 

 it appeared that the freezing extended outwardly from the pipes 

 about three-fourths as far as within the circle. 



The material was mostly loosened by picks and chisel bars. 

 Powder was used for blasting for a considerable time, but this was 

 discontinued for fear the concussion might injure the pipes or frac- 

 ture the wall. The material was hoisted out by an iron bucket, 

 which also took out the water that stood in the unfrozen centre. 

 There was no appreciable inflow of water until the excavation had 

 reached nearly to the ledge, when a small amount was noticed. 



On reaching the ledge, it was discovered that it was so fissured 

 and disintegrated as to allow water to come in under the frozen 

 wall at a corner in the vicinity of one of the pipes that did not ex- 

 tend to the ledge. The shaft was allowed to flood, water being 

 pumped into it at the same time to prevent as much as possible the 

 flow of water through the opening. An eight-inch freezing pipe 

 was put in place in the shaft, the foot being directly at the opening, 

 and earth was piled around it, the purpose being to freeze the 

 leak off. Then cold brine was circulated through the whole system 

 of freezing pipes for ten days uninterruptedly, when the water was 

 pumped out, and the seam was found to be quite closed ; but there 

 was still a small amount of percolation through the ledge, requiring 

 occasional pumping to clear the shaft ; ice had collected several 

 inches thick on the side shaft, and several feet in the corner, 

 where the extra freezing pipe was placed. 



The work of removing the earth which had been thrown in and 

 the clearing up of the bottom continued for two weeks, when the 

 water from the ledge increased at such a rate that it was decided 

 to lay short auxiliary freezing pipes against the leaks and freeze 

 the ledge itself. This was done, the shaft was flooded again, and 

 the brine circulated thirty days. When the water was pumped 

 out, the leakage was found to be small, and excavation was pro- 

 ceeded with. The soft, shaly rock was removed till a hard bear- 



