“I next proceeded to the Bay, and took two moulds from the footprints of the man with the nailed shoes, a right and a left.  Here is a cast from the mould, and it shows very clearly that the man was walking backwards.”

“How does it show that?” asked the magistrate.

“There are several distinctive points.  For instance, the absence of the usual ‘kick off’ at the toe, the slight drag behind the heel, showing the direction in which the foot was lifted, and the undisturbed impression of the sole.”

“You have spoken of moulds and casts.  What is the difference between them?”

“A mould is a direct, and therefore reversed, impression.  A cast is the impression of a mould, and therefore a facsimile of the object.  If I pour liquid plaster on a coin, when it sets I have a mould, a sunk impression, of the coin.  If I pour melted wax into the mould I obtain a cast, a facsimile of the coin.  A footprint is a mould of the foot.  A mould of the footprint is a cast of the foot, and a cast from the mould reproduces the footprint.”

“Thank you,” said the magistrate.  “Then your moulds from these two footprints are really facsimiles of the murderer’s shoes, and can be compared with these shoes which have been put in evidence?”

“Yes, and when we compare them they demonstrate a very important fact.”

“What is that?”

“It is that the prisoner’s shoes were not the shoes that made those footprints.”  A buzz of astonishment ran through the court, but Thorndyke continued stolidly:  “The prisoner’s shoes were not in my possession, so I went on to Barker’s pond, on the clay margin of which I had seen footprints actually made by the prisoner.  I took moulds of those footprints, and compared them with these from the sand.  There are several important differences, which you will see if you compare them.  To facilitate the comparison I have made transparent photographs of both sets of moulds to the same scale.  Now, if we put the photograph of the mould of the prisoner’s right shoe over that of the murderer’s right shoe, and hold the two superposed photographs up to the light, we cannot make the two pictures coincide.  They are exactly of the same length, but the shoes are of different shape.  Moreover, if we put one of the nails in one photograph over the corresponding nail in the other photograph, we cannot make the rest of the nails coincide.  But the most conclusive fact of all—­from which there is no possible escape—­is that the number of nails in the two shoes is not the same.  In the sole of the prisoner’s right shoe there are forty nails; in that of the murderer there are forty-one.  The murderer has one nail too many.”

There was a deathly silence in the court as the magistrates and Mr. Bashfield pored over the moulds and the prisoner’s shoes, and examined the photographs against the light.  Then the chairman asked:  “Are these all the facts, or have you something more to tell us?” He was evidently anxious to get the key to this riddle.