Sentence:  I let up on the accelerator , only to <WSD>gradually</WSD> reach again the 60 m. p. h. which would , I hoped , overhaul Herry and the blonde , and as there were cars whose drivers apparently had something more important to catch than had I , Mrs. Major Roebuck settled down to practicing on Corporal Johnson the kittenish wiles she would need when making her duty call on Colonel and Mrs. Somebody in Sante Fe . 
Meaning of the word marked with <WSD> tags:  in a gradual manner