33

C.—2.

5--C. 2.

Effect of Varting Conditions. The wide variations in the working-conditions underground will naturally cause considerable differences in the counts obtained from the various samples, even from those taken at short intervals at the same place. Among the more important of the causes affecting the sample obtained may he ineiil ioned— (a.) The air-space. In confined places like winzes and dead ends Ihe eoimls are likely to be higher than in open slopes, &o. (//.) Quantity of air circulating. The larger the volume of air circulating, the more the dusl is diluted ami the smaller the count. (c.) Eddy ourrents. These are especially produced by the exhaust from the machine drills, and the counts will vary considerably according to the position where the instrument is held. (d.) Character of rock drilled. Hard rock as a rule produces more dust-particles than softer rock, even though it is not drilled so fast, (c.) Hale of drilling. Oilier things being equal, the number of particles of dust projected into the air should lie approximately proportional to the rate of drilling. (/.) Type of hole. It seems probable that a hole bored below the horizontal will produce less dust than an " upper," as the dust has a, greater chance of being killed by the water in the hole. (g.) Humidity of the air. This will affect the amount of dust in suspension in levels and return airways. Saturated air rapidly precipitates dust, and this accounts for the almost entire absence of dust, in the upcast shafts. (/(..) Dryness of the rock. Where the rock or country being drilled is moist, less dust is given off. Reference has already been made to I lie necessity for thoroughly wetting dry working-places. MIORO-PHOTOGRAPHS. A few micro-photographs accompanying this reporf will afford ocular evidence of the nature of the dust produced in several operations. The magnification is in all cases forty diameters. The photographs are not necessarily of average spots —in fact, represent some of the worst samples obtained during the course of (he work.

Summary of Results.

Samples: When- taken. Number of Samples »"S 8 fill » O h >oA3, < CO O Ol HP la M ° toM . a o Remarks. Upoast shafts Levels Levels, vicinity of shrin kage stopes Stopes (no drilling in progress) .. Stopes, hiiininer-di'ill, n-xiiil waterfeed Ditto Winzes (no drilling in progress).. Winzes, hammer-drill, axial waterfeed Winzes, 15 minutes after firing .. Rises (no drilling in progress) .. Drives, reciprocating-drill, water■i'' 1 ' Drives, reciprocating-drill, axial water-feed Ditto Drives, hammer-drill, water-jet.. Drives, hammer-drill, axial waterfeed Ditto 16 8 2 8 3 10 25 54 47 02 100 94 90 98 97 Air saturated. Intake air. After filling trucks. Drawing off shrinkage stope. Holman stoper. II 50 48 147 98 95 98 I ngersoll Leyncr 248. 6 3 Denver " Clipper." 6 3 15 97 29 91 91 98 97 Ventilated by compressed air. Holman. 3 81 9G Edgar. 2 11 10 125 127 117 96 92 94 Ingvrsoll E. 33. [ngersoll " dry " stoper. Ingersoll " wet " stoper. ,, 30 27 4 76 177 485 98 96 99 [ngersoll Leyner 248. Waiigh's " Tubro." Dry place not hosed down before working. Drives, after shovelling Drives, face 10 minutes after blasting. Drives, return air 10 minutes after blasting Rise, 15 minutes after blasting.. 6 3 135 84 90 9G Dust, cleared by fan. 3 744 93 Blasting dust. 4 990 99 Blasting dust. Waihi School of Mines, 18th Augusi, 1920. A. H. V. Mohoan.