It really seemed as if the man's mechanical merriment were no part of himself.

He was, in fact, gaining time to propound an explanation which he did not believe in the least, but which happened to be almost the exact truth.
He answered with an air of ironic indifference.
'Well, you know, I wouldn't go in for the detective line, if I were you, Harris.

You aren't subtle enough for it.

You jump too quickly at conclusions which have nothing to do with the main point.

In fact, you're a fool, Harris--a damned fool.' Harris' puzzled expression turned to one of extreme indignation.