Sentence:  ...whoever takes the trouble of observing how the water runs longitudinally in the ruts on a convex road, although the <WSD>declivity</WSD> down the sides be incomparably greater than in the direction which it is compelled to take in the ruts, will soon see the propriety of constructing roads so as to have the water rim length-ways upon them, instead of attempting to gain a <WSD>declivity</WSD>, by making it run from the middle to the sides. 
Meaning of the word marked with <WSD> tags:  The downward slope of a hill; the downward slope of a curve.