The Habas tunnel through quicksand, between Dax and Ramoux, France, cost $118.50 per lin. ft. The cost of the Boston subway was $342.40 per lin. ft. The Severn and Mersey tunnels, constructed through rock under water, cost respectively $208.38 and $263 per lin. ft. The First Thames Tunnel, driven by Brunel’s shield, cost $1661.66 per lin. ft. The Hudson River and St. Clair River tunnels, excavated through soft ground by means of shields and compressed air, cost respectively $305 and $315 per lin. ft. The Blackwall double-track tunnel under the River Thames, which is the largest tunnel ever built by the shield system, cost $600 per lin. ft. In making estimates of the cost of projected tunnel work based on the cost of tunnels previously constructed through similar materials, it is important to keep in mind the date and location of the work used as the basis for calculations. For example, a tunnel excavated in Italy, where labor is very cheap, will cost less than one excavated in America, where labor is dear, all other conditions being the same. Other reasons for variation in cost due to difference of date and location of construction will suggest themselves, and should be taken into full consideration in estimating the cost of the new work. =Time.=--The time required to excavate a tunnel depends upon the character of the material penetrated and upon the method of work adopted. Tunnels driven through soft ground by hand require about the same time to construct as tunnels driven through hard rock by the aid of machinery. Tunnels can be driven through hard rock at about as great a speed as through soft or fissured rock, chiefly because the work of blasting is more efficient in hard rock, and because no time is required in timbering. The following table shows the average rate of progress in different parts of the tunnel excavation through both hard and soft materials in feet per month:-- +---------------+--------------------+--------------------+-----------+ | QUALITY | HEADING. | EXCAVATION |ENLARGEMENT| | OF SOIL. | | OF SHAFTS. |OF PROFILE.| | +----------+---------+---------+----------+-----------+ | | By hand. | By | By hand.| By | By hand. | | | | machine.| | machine. | | +---------------+----------+---------+---------+----------+-----------+ |Very loose soil|16.7- 26.8| | 6.6-16.7| | 6.6- 16.7| |Loose soil |33.4-100 | |16.7-33.4| | 16.7- 33.4| |Soft rock |66.8 |233.8-334|33.4-66.8|66.8-132.6| 33.4- 50 | |Hard rock |50 - 66.8|233.8-334|33.4-50 |66.8-132.6| 66.8-100 | |Very hard rock |33.4 |233.8-334|16.7-33.4|66.8-132.6| 66.8-100 | +---------------+----------+---------+---------+----------+-----------+