And here is as good a place, perhaps, as any other in this chapter, to say that true engineering is the economical adaptation of the means and opportunities existing, to the end desired. Civil engineering was defined, by one of the greatest of England's engineers, as "the art of directing the great sources of power in nature for the use and convenience of man," and that definition was adopted as a fundamental idea in the charter of the English Institution of Civil Engineers. But the development of engineering-works in America has been effected successfully by American engineers only because they have appreciated another side of the problem presented to them. A past president of the American Society of Civil Engineers, a man of rare judgment and remarkable executive ability, the late Ashbel Welch, said, in discussing a great undertaking proposed by an eminent Frenchman: "That is the best engineering, not which makes the most splendid, or even the most perfect, work, but that which makes a work that answers the purpose well, at the least cost." And it may be remarked, as to the project which he was then discussing, that after a very large expenditure and an experience of eight years since that discussion, the plans of the work were modified and the identical suggestions made by Mr. Welch of a radical economical change were adopted in 1888.[8] Another eminent American engineer, whose practical experience has been gained in the construction and engineering supervision of more than five thousand miles of railway, said, in his address as President of the American Society of Civil Engineers: "The high object of our profession is to consider and determine the most economic use of time, power, and matter." [Illustration: Perspective View of St. Gothard Spiral Tunnels, in the Alps.] That true economy, which finally secures in a completed work the best results from the investment of capital, in first cost and continued maintenance, is an essential element in the consideration of any really great engineering feat. * * * * * The difficulties involved in the construction of a tunnel, after the line and dimensions have been determined, depend generally upon the nature of the material found as the work advances. Solid rock presents really the fewest difficulties, but it is seldom that tunnels of considerable length occur without meeting material which requires special provision for successful treatment. In some cases great portions of the rock, where the roof of the tunnel is to be, press downward with enormous weight, being detached from the adjacent mass by the occurrence of natural seams. At other places soft material may be encountered, and the passage then is attended with great difficulty. Temporary supports, generally of timber, and of great strength, have often to be used at every foot of progress to prevent the material from forcing its way into the excavation already made.