A little more than fifty years ago petroleum, or rock oil, was discovered near Pittsburgh, and although oil has since been found in many other places, Pittsburgh is still one of the great centers for this product. Crude petroleum as it comes from the earth is a liquid, formed from the decay of plants and animals long ago buried underground. It is obtained by sinking wells, or pipes, into oil-bearing rock, which is very porous. Sometimes the pipes are sunk a quarter of a mile deep. The average yield is from 50 to 75 barrels a day, and occasionally a pipe well is found which yields as high as 1000 barrels. Sometimes a well stops flowing. Then the oil must be pumped from the earth or else forced out by the explosion of dynamite. Such a well is spoken of as a "shot well." When a well is shot, a vast column of oil is thrown into the air, just as water is thrown up in a geyser or hot spring, by the action of gases under ground. Pittsburgh makes great storage tanks for the oil, as well as apparatus for drilling wells, and supplies these not only to our own country but to every foreign land in which oil is found. When petroleum is heated it gives off vapors, varying according to the heat. These vapors are then condensed and form many products which are now in every-day use, such as kerosene, gasoline, naphtha, and benzine. Vaseline is what remains in the vats after heating the petroleum. Paraffin is another product. Pittsburgh manufactures all these and supplies them to the world. The discovery of natural gas about twenty-five years ago, and its use as a fuel, attracted the attention of the world to Pittsburgh as a center of cheap fuel. Natural gas is found in and around oil fields, so it is supposed that the gas and the oil have the same origin. The porous rock in which the gas is found is usually covered with clay rock, or shale, which prevents the gas from escaping. Natural gas, like petroleum, is obtained by sinking pipes. When the gas is reached, it rushes out with great force. Large quantities of it were formerly used in Pittsburgh's glass factories and iron works, but its greatest use to-day is for lighting and heating. The city of Pittsburgh stretches for 7 miles along the Allegheny, about the same distance on the Monongahela, and entirely covers the space between. The city of Allegheny, across the Allegheny River, has recently been annexed, thus giving Pittsburgh an area of 38 square miles. The two cities, with the river between, remind us of Brooklyn and Manhattan. [Illustration: WOOD STREET AT SIXTH AVENUE IN 1902] The city's water supply is taken from the Allegheny River and is purified in the largest single filtration plant in the world.