steepen

verb

1. (transitive)To make steeper.

   “With a rare and beautiful ease one can now ring up a boffin, as I did the other day, and say: "As a price for not opposing our Parliamentary Powers for a new marshalling yard, the Council at X demands that the bridge over X Lane shall have 16 ft. 6 in. headroom. This means steepening our gradient from 1 in 70 to 1 in 65 for half a mile on a 20-chain curve. What difference will this make to the loads of Type "2", "3" and "4" diesels please?". Back comes the answer.”

2. (intransitive)To become steeper.

   “As the way steepened […] I could detect in the hollow of the hill some traces of the old path.”

   “Soon after Crowhurst, the down grade steepens to 1 in 100 as the line descends the side of a valley, at the mouth of which a glimpse of the sea can be caught—the only one.”

   “Immediately beyond Churn Lane the climb begins at 1 in 78, steepening to 1 in 66. Speed soon falls and a number of heavy hop-pickers' specials have come to grief here.”