To the English engineer, Peter Barlow, above mentioned, must be given the credit of bringing into use the first really serviceable circular shield for soft ground tunneling. In 1863 he took out a patent for such a shield with a cylindrical cast iron lining for the completed tunnel. Of course James Henry Greathead very materially improved the shield, so much so indeed that the present system of tunneling by means of circular shields is called the Greathead not the Barlow system. Greathead and Barlow entered into a partnership in 1869. They constructed the tunnel under the Tower of London 1,350 feet in length and seven feet in diameter which penetrated compact clay and was completed within a period of eleven months. This was a remarkable record in tunnel building for the time and won for these eminent engineers a world wide fame. From thenceforth their system came into vogue in all soft soil and subaqueous tunneling. Except for the development in steel apparatus and the introduction of electricity as a motive agent, there has not been such a great improvement on the Greathead shield as one would naturally expect in thirty years. The method of excavating a tunnel depends altogether on the nature of the obstruction to be removed for the passage. In the case of solid rock the work is slow but simple; dry, hard, firm earth is much the same as rock. The difficulties of tunneling lie in the soft ground, subaqueous mud, silt, quicksand, or any treacherous soil of a shifting, unsteady composition. When the rock is to be removed it is customary to begin the work in sections of which there may be seven or eight. First one section is excavated, then another and so on to completion. The order of the sections depends upon the kind of rock and upon the time allotted for the job and several other circumstances known to the engineer. If the first section attacked be at the top immediately beneath the arch of the proposed tunnel, next to the overlying matter, it is called a heading, but if the first cutting takes place at the bottom of the rock to form the base of the tunnel it is called a drift. Driving a heading is the most difficult operation of rock tunneling. Sometimes a heading is driven a couple of thousand feet ahead of the other sections. In soft rock it is often necessary to use timber props as the work proceeds and follow up the excavating by lining roof and sides with brick, stone or concrete. The rock is dislodged by blasting, the holes being drilled with compressed air, water force or electricity, and, as has been said, powerful explosives are used, nitroglycerine or some nitro-compound being the most common. Many charges can be electrically fired at the same time. If the tunnel is to be long, shafts are sunk at intervals in order to attack the work at several places at once. Sometimes these shafts are lined and left open when the tunnel is completed for purposes of ventilation.