■It is culatea that at the present rate ol consumption, the opal oí England will last 700 years. The deptb of some of the Ëriglisb coal mines, is enormous. The pit at Duckerfield is 2,504 feot deep ; that at Ponrlleton is 2,122. The canne! coa) of Wigan, is bronght frorn 1,773 feet beio-w the surface. The Delcouth tin mine is more than 1,800 in depth, while the Tresaveen copper mine ia 2,180 Severa) of the mines of Cornwall run neiirly half a mile under tho Atl mtic. The average heat at the Burface of tho oarth in the latitude of England, is 50 degrees, while the increase of temperatne from the surfaca downward, is óne degree for every 44 feet. In the bottom of the deepest mine mentioned ábove, the heat, therefore, would be 107 degrees.