Mr. A. S. E. ACKERMANX noticed'that the Author gave a forMr. Ackermula for calculating the quantity of air required per minute to ventilate a tunnel, but he thought he should have explained how it had been deduced. If the following method had been adopted, it would be seen that no account had been taken of the air required for the actual combustion of the coal. This amounted to nearly 1 0 per cent. of the whole, and in any case should have been mentioned. In the formation of CO, the equations C+O, = CO, (12 + 2 X 161 = 44) (where the second equation referred to the atomic weights of the elements) showed that 12 lbs. of carbon combined with 32 lbs. of oxygen to form 44 lbs. CO,, or 1 lb. of carbon gave lbs. of CO,. At atmospheric pressure and a temperature of say 60" F., about 8 . 6 feet of CO, weighed 1 lb., hence 1 lb. of carbon gave X 8 6 = 31 5 cubic feet of CO,. The coal was not all carbon; hence 31 5 cubic feet of CO, was not obtained per lb. of coal ; but, taking a high percentage of carbon, 29 cubic feet CO, was given per lb. of coal. The permissible amount of CO, in the tunnel air was taken at 2 O = 3&, hence for every cubic foot of CO, 500 cubic feet of air must be supplied ;hence 29 X 500 cubic feet of air per lb. of Goal were required; 32 lbs. of coal were burnt per mile, therefore 29 x500 X 32 cubic feet of air were required per train-mile.