Here, for instance, is a letter which has been opened after being fastened up--apparently by the aid of steam.

The envelope is soiled and rubbed, and smells faintly of stale tobacco, and has evidently been carried in a pocket along with a well-used pipe.

Why should it have been opened?
On reading it I perceive that it should have reached me two days ago, and that the date has been skilfully altered from the thirteenth to the fifteenth.

The inference is that my correspondent has a highly untrustworthy clerk." "But the correspondent may have carried the letter in his own pocket," I objected.
"Hardly," replied Thorndyke.

"He would not have troubled to steam his own letter open and close it again; he would have cut the envelope and addressed a fresh one.