for a small fraction of this time, required for plant modifications.
From numerous tests average efficiencies of removal were sulphur oxides 97 to 99 per cent., nitrogen oxides 60 to 70 per cent., hydrochloric acid 90 to 93 per cent., grit and dust from P.F. boiler 97 to 98 per cent., dust from P.F. boiler left in flue gas after having passed through a well known wetted tube deduster 90 to 93 per cent.
R. H. 0. World's Largest Tunnel for Bay Bridge.-C. W. GEIGER describes in the Excavating Engineer for April, 1935, this tunnel on Yerba Buena Island. It is a part of the San Francisco-Oakland Bay Bridge, which, supported by high massive piers, reaches out from San Francisco, traverses the island through what is stated to be the world's largest bore highway tunnel, and then, by means of more high piers, touches the mainland at Oakland.
Its "two deck" construction provides, on the upper deck, six lanes for fast moving automobiles and, on the lower deck three lanes for trucks and two interurban-railway tracks. The tunnel is 540 ft. in length, 78 ft. wide and 67 ft. high. While massive concrete and steel walls will take up much of this great width, the completed tunnel will have a 56 foot clear roadway and a total height of about 58 ft.
This tunnel is so large that it was built by boring five pilot tunnels through the island and then breaking them down into the full bore. Two bottom headings at the base of each of the walls were put through from the west portal to the east portal of the tunnel, two more were tunneled through the side walls above these, and the fifth pilot tunnel was drilled at the crown. R. H. 0.