nine o'clock
This gave the triplets a good chance to carry out their designs . But Bob Holliday and Jack , good friends to the teacher , thought that it would be fun to watch the conspirators and defeat them . So , when they saw Mr. Williams going to Dr. Lanham 's , they stationed themselves in the dark alley on the side of the street opposite to Riley 's and took observations . Mr. Williams had a habit of leaving Dr. Lanham 's at exactly nine o'clock , and so , just before nine , the three came out of Riley 's yard , and proceeded in the darkness to the fence of Lanham 's dooryard . Getting the trunk of one of the large shade-trees between him and the plotters , Jack crept up close enough to guess what they were doing and to overhear their conversation . Then he came back to Bob . `` They are tying a string across the sidewalk on Lanham 's side of the alley , I believe , '' whispered Jack , `` so as to throw Mr. Williams head foremost into that mud-hole at the mouth of the alley . ''