He continued to adjust the machine which he had brought.  Langhorne watched minutely, but did not say anything.

“Bertillon used to call this his mechanical burglar detector,” continued Kennedy.  “As you see, this frame carries two dynamometers of unequal power.  The stronger, which has a high maximum capacity of several tons, is designed for the measurement of vertical efforts.  The other measures horizontal efforts.  The test is made by inserting the end of a jimmy or other burglar’s tool and endeavouring to produce impressions similar to those which have been found on doors or windows.  The index of the dynamometer moves in such a way as to make a permanent record of the pressure exerted.  The horizontal or traction dynamometer registers the other component of pressure.”

He pressed down on the machine.  “There was a pressure here of considerably over two tons,” he remarked at length, “with a very high horizontal traction of over four hundred pounds.  What I wanted to get at was whether this could have been done by a man, woman, or child, or perhaps by several persons.  In this case, it was clearly no mere fake to cover up the opening of the door by a key.  It was a genuine attempt.  Nor could it have been done by a woman.  No, that is the work of a man, a powerful man, too, accustomed to the use of the jimmy.”

I fancied that a shade of satisfaction crossed the otherwise impassive face of Langhorne.  Was it because the Bertillon dynamometer appeared at first sight to exonerate Betty Blackwell, at least so far, from any connection with the crime?  It was difficult to say.

Important though it was, however, to clear up at the start just what sort of person was connected with the breaking of the door I could not but feel that Kennedy had some purpose in deferring and minimizing for the present what, to me at least, was the greater mystery, the entering of the safe itself.

He was still studying and comparing the marks on the door and the record made on the dynamometer, when the office telephone rang and Langhorne was summoned to answer it.  Instead of taking the call in his own office, he chose to answer it at the switchboard, perhaps because that would allow him to keep an eye also on us.

Whatever his purpose, it likewise enabled us to keep an ear on him, and it was with surprise which both Kennedy and I had great difficulty in concealing, that we heard him reply, “Hello—­yes—­ oh, Mrs. Ogleby, good-morning.  How are you?  That’s good.  So you, too, read the papers.  No, I haven’t lost anything of importance, thank you.  Nothing serious, you know.  The papers like to get hold of such things and play them up.  I have a couple of reporters here now.  Heaven knows what they are doing, but I can foresee some more unpaid advertising for the firm in it.  Thank you again for your interest.  You haven’t forgotten the studio dance I’m giving on the twelfth?  No—­that’s fine.  I hope you’ll come, even if Martin has another engagement.  Fine.  Well-good-bye.”