The great difficulty in working coal, should these upper seams fail, is not only the increase of cost in sinking further down, but the increased heat to be worked in. At 2000 feet the mine will increase in heat 28°, at 4000, 57°; to this must be added the constant temperature of 50° 5', so that at 2000 feet it would be 78° 5', and at 4000, 107° 5' Fahr. By actual trial on July 17, 1857, in Duckingfield Pit, the temperature at 2249 feet was 75° 5'. From this it may be conceived in what great heat the men have to work, and the work is very hard. One may fancy from this what can be endured, but it would be next to impossible to work in a greater temperature. I can speak upon this from actual experience, as when down the Lady Londonderry Pit the temperature was 85°, and here the men worked naked. Another great source of expense and anxiety lies in keeping up the roof, as, from the excessive pressure, the roof and floor are always inclined to come together, and props must therefore be used, and these in some pits cost as much as £1500 a year. To digress for a moment, an amusing story is told of Grimaldi, the celebrated clown, when paying a visit to a coal-pit. Having gone some way through the mine, a sudden noise, arising from the falling of coal from the roof, caused him to ask the reason of the noise. "Hallo!" exclaimed Grimaldi, greatly terrified, "what's that?" "Hech!" said his guide, "it's only a wee bit of coal fallen down--we have that three or four times a day." "Then I'll thank you to ring for my basket, for I'll stop no longer among the wee bits of falling coal." This "wee bit" was about three tons' weight. A large proportion of the sad accidents in coal-mines is caused by these falls of the roof, which give no warning, but suddenly come down and crush to death those who happen to be near. MODE OF WORKING.