It should, however, be noted that many engines at the present day do not consume 4 lbs.
If therefore ventilating machines lose 40 and utilise 60 per cent, of the engine power, the depths that are necessary to render furnace ventilation as economical as such ventilating machines in the consumption of fuel are as stated above.  It should, however, be noted that many engines at the present day do not consume 4 lbs.  of coal per horse-power per hour, and hence the economy of fan ventilation is more than that shown by the table.