Here the contours are such that had the road deflected to the west or the east in order to avoid the highest summit, it would have been compelled either to a very long detour (involving in any case _nearly_ as high a climb) or to a series of steep and profound ups and downs over the spurs of the mountain. The line taken from the Moselle to Belfort on the other side goes within a few feet of the highest point on the hill, and is yet the line of least effort from one point to the other. It is an excellent example of the way in which the formula of _minimum effort_, when it is thought out, may be quite different from what mere habit would have produced. CHAPTER IV THE OBSTACLE OF VEGETATION _The Special Expenditure due to Forest: Roads which Skirt Woodlands: Roads which have been Deflected by Forest: Proximity of Material as a Final Main Cause Modifying the Trajectory of a Road: Cost of Transporting Material and its Effects in Ancient and Modern Times._ i The obstacle of vegetation, which is our next cause modifying the trajectory of a road, is two-fold. There is the obstacle presented by forests or permanent vegetation (which includes in some climates very high grasses) and the obstacle presented by intermittent growths. We are not, in this country and in modern times, well acquainted with the obstacle of vegetation to a road and with the modification of trajectory which it imposes. We have no _large_ forests left: we have, in common with all Northern Europe, no exuberance of growth. The dense population and very high road mileage of modern England have put this factor in the development of communication out of sight, and it is so unrecognized that the mention of it here may seem superfluous. But it is still a grave element in the calculation of a road even in the European world, and a graver one in the new countries. And it has had its part in framing our own system in its earlier stages. In damp tropical countries it is all important, and even in temperate climates where large forests exist it has its place.