0.-4

22

To reduce excessive temperatures and to cause an appreciable current to circulate through the heated stopes of the mines an increased volume of air is necessary, but the extent to which underground temperature may be reduced by this means is somewhat limited, as our investigations have proved. The rate of increase in temperature of the outside air when conveyed into deep mines is demonstrated by the following measurements taken by us. During our inspection of the Thames mines we ascertained by actual measurement that atmospheric air which entered a Boots blower at the surface, and was forced at a velocity of 10 ft. per second through a thin gaivanized-iron 22 in. air-pipe down a vertical shaft 1,000 ft. in depth, and along a crosscut 670 ft. in length, increased in temperature in the space of 2 minutes 47 seconds from 55° to 76° Fahr., being at the rate of 7-64° Fahr. increase per minute. The temperature of the air surrounding the pipe did not exceed 81° Fahr. At the Waihi-Grand Junction Mine we found that the temperature of the air, while passing from the top to the bottom of a vertical downcast shaft 955 ft. in depth, increased at the time of measurement from 51° to 56° Fahr., the velocity of the air being at the rate of 16 ft. per second. In this case the temperature of the air in the shaft increased at the rate of 5° Fahr. per minute. The return air in the fan passage at the surface of this mine was 69° Fahr. (completely saturated). From the foregoing examples it will be noted that in the North Island of New Zealand, where the outside temperature is equable (the mean surface temperature at Waihi being 55-7° Fahr.), the temperature of the air descending into the mine will have attained that of the rock-surfaces within seven minutes of entering the shaft. In discussing the effect of high temperatures on miners the British Koyal Commission on Mines (1909) reported as follows : — Ihe influence of high temperatures on men is intimately dependent on the moisture in the air, and also on its motion, for the greater the dryness of the air and the greater its motion the more rapidly does warm air carry off the heat of the body and thus neutralize the ill effect of the heat. The existing evidence indicates that in still and saturated air continuous hard work is practically impossible at temperatures exceeding about 80° Fahr., even when men are stripped to the waist, and that when the air-temperature is higher than this the result is the same if the wet-bulb temperature rises above 80° Fahr. A temperature of 100° or 110° Fahr., with the air so dry that the wet-bulb temperature is only 80°, is thus no worse than air completely saturated with moisture at 80° Fahr. In other words, it is the wet-bulb temperature, and not the actual temperature of the air, tiiat matters to a man when the air-tem-perature is high. In moving air, however, a somewhat higher wet-bulb temperature can be borne than in still air. At wet-bulb temperatures exceeding about 80° Fahr. the amount of continuous work which a man is capable of doing without serious rise of bodily temperature rapidly falls off and becomes practically nothing at 90° wet bulb. As a result of our investigations and deductions from the foregoing evidence we are of opinion that, to secure a continuous distribution of the air in metal-mines adequate to reduce the temperature in hot working-places within reasonable limits, a standard or fixed temperature of 80° Fahr., wet bulb, should not be exceeded, unless it is not reasonably practicable to maintain the air at such standard, in which case a reduction of the hours of employment should be provided for. In three of the Australian States fixed standards of temperature are nowprovided for by law, as follows : — West Australia : Temperature not to exceed 80° Fahr. wet bulb, or 87° Fahr. dry bulb, without permission of Inspector in case of impracticability. Victoria : Temperature not to exceed 83° Fahr., wet bulb, without permission of Inspector in case of impracticability. Queensland : A reasonable limit of humidity shall be maintained— viz., at a temperature of 85° Fahr. by the dry bulb the wet bulb should be at least 5° less, and in case of a higher temperature from 7° to 8° less. We have the honour to make the following recommendations in connection with the ventilation of metal-mines :—