ON ROADS AND BRIDGES. 125 



upon the ground, that it is not always so, for it may pass over so many 

 elevations and depressions that it is actually longer than a line traced 

 near it and avoiding these irregularities. 



If we have a hill of a hemispherical form, like half of a globe, 

 jilaced upon a table, the distance from one side to the other over the 

 top will be precisely the same as the distance around it at its base, and 

 we should have the disadvantage of going up on one side and down 

 on the other, instead of keeping a level road around. 



Although this principle seems a sinrple one, we find it continually 

 disregarded, there being frequent cases where common roads pass 

 directly over a high point with lower ground within a few feet on 

 either side of them. In fact, in any country other than a perfectly 

 level one, a road which keeps a straight direction for mile after mile, as 

 many of our turnpikes do, must necessarily be badly located, since 

 advantage has evidently not been taken of the natural features of the 

 surface. 



We must bear in mind the fact that the force required to draw a 

 well made wagon in good order over a smooth level road is very small 

 compared with the absolute weight of the wagon and load. On a good 

 turnpike about one fiftieth of the load,* that is a tractive force often 

 pounds will move a load of five hundred pounds, the only resistance 

 being from friction of the axles and from the minute obstacles of the 

 surface. Under such circumstances, the horse's power is applied most 

 economically. 



When a horse attempts to move a load up an inclined plane, however, 

 in addition to overcoming the friction, he has also to raise a part of 

 the weight of the wagon, according as the inclination is more or less 

 great. Now, if the two places connected by the road are on the same 

 level, all lifting of the load up inclinations only to let it down again 

 on the other side will be so much power expended uselessly. Increas- 

 ing the length of a road, therefore, to avoid hills, is in most cases an 

 economy to the traveler. Of course, the exact amount of increase, or 

 the equation of grades and distances, as it is called by the engineer, 

 must be a matter of calculation based upon experiment and observation. 



A considerable deviation can be made to the right or left of a straight 

 line joining two points without materially increasing the length of the 

 road. For example : if the two points be ten miles apart, we may 

 deviate a whole mile at the middle of the distance, to either side, with- 

 out increasing the length of the path traveled the fifth of a mile. 



Having these general principles to guide him, the engineer, in 

 locating a road, should first make a thorough examination, on foot or 

 on horseback, of the whole country lying between the points to be con- 

 nected. He should collect all the maps of the region that he can find, 

 and he should gather from the inhabitants information on various 

 subjects: such as, where low places exist in the ridges ; what jjoints 

 are particularly free from, or filled up with, snow in the winter ; what 

 places are remarkably exposed to the wind; and particularly ascertain 

 the height and boundaries of all the streams during the highest freshets 

 that have been known, so that no part of the road or bridges may be 

 •exposed to danger from a rise of the water. In a rather small region, 



* Poncelet. Morin. 



