Another periodical process that must be carried on in the intervals of drilling is that of lining the hole with casing, in order that water and caving strata may be cased off before the oil sands are reached. After a well is operating, the lower part of the casing may rust through, causing leakage. To meet this difficulty an inner casing is put in place with a casing shoe, on the outside of which is lead or other soft material which expands under pressure from above to make a snug fit. Not infrequently, it is necessary to decrease the size of the hole with packers in this way four or five times, though it is kept as large as is practicable all the way down. When oil is struck it is sometimes suddenly driven to the surface by imprisoned gas, and another gusher, a comparatively common phenomenon in Mexico, is recorded. But if this condition does not arise, tubing and pump are inserted and the oil is drawn to the surface. Not infrequently, however, the oil sands at the outset do not yield an adequate flow and in a great number of cases what is known as “shooting” with nitro-glycerine, an interesting and once dangerous process, is resorted to. In the early days before oil production had been reduced to scientific formulas the obtaining of crude was often attended with serious hazards to life. Ignorance of the properties of petroleum also created imaginary dangers for the pioneers. In 1860 the people of Western Pennsylvania were thrown into a panic by the proposal of a stranger, claiming to be a European scientist, to shoot a white-hot bolt into the bowels of the earth through an iron pipe driven to a great depth for the purpose. By the ignition of inflammable gases thought to exist in the great cavities beneath the earth’s crust the promoter expected to produce a sufficient explosion to lay bare the subterranean reservoirs of oil. The Pennsylvania populace, instead of viewing this proposal with the apathy usually accorded to the first essays of inventive minds, possessed sufficient imagination to picture the possible results, and were so convinced that the alleged scientist minimized the possibilities of his project that they selected a small but determined committee to lynch him. Because he threatened to undermine not merely the foundations of society but the ground on which society subsisted, he was taken into custody by the authorities and solemnly warned to desist.