“Extraordinary,” murmured Carlyle.

“Not very,” said Carrados.  “If someone dipped a stick in treacle and wrote ‘Rats’ across a marble slab you would probably be able to distinguish what was there, blindfold.”

“Probably,” admitted Mr. Carlyle.  “At all events we will not test the experiment.”

“The difference to you of treacle on a marble background is scarcely greater than that of printers’ ink on newspaper to me.  But anything smaller than pica I do not read with comfort, and below long primer I cannot read at all.  Hence the secretary.  Now the accident, Louis.”

“The accident:  well, you remember all about that.  An ordinary Central and Suburban passenger train, non-stop at Knight’s Cross, ran past the signal and crashed into a crowded electric train that was just beginning to move out.  It was like sending a garden roller down a row of handlights.  Two carriages of the electric train were flattened out of existence; the next two were broken up.  For the first time on an English railway there was a good stand-up smash between a heavy steam-engine and a train of light cars, and it was ‘bad for the coo.’”

“Twenty-seven killed, forty something injured, eight died since,” commented Carrados.

“That was bad for the Co.,” said Carlyle.  “Well, the main fact was plain enough.  The heavy train was in the wrong.  But was the engine-driver responsible?  He claimed, and he claimed vehemently from the first, and he never varied one iota, that he had a ‘clear’ signal—­that is to say, the green light, it being dark.  The signalman concerned was equally dogged that he never pulled off the signal—­that it was at ‘danger’ when the accident happened and that it had been for five minutes before.  Obviously, they could not both be right.”

“Why, Louis?” asked Mr. Carrados smoothly.

“The signal must either have been up or down—­red or green.”

“Did you ever notice the signals on the Great Northern Railway, Louis?”

“Not particularly, Why?”

“One winterly day, about the year when you and I were concerned in being born, the engine-driver of a Scotch express received the ‘clear’ from a signal near a little Huntingdon station called Abbots Ripton.  He went on and crashed into a goods train and into the thick of the smash a down express mowed its way.  Thirteen killed and the usual tale of injured.  He was positive that the signal gave him a ‘clear’; the signalman was equally confident that he had never pulled it off the ‘danger.’  Both were right, and yet the signal was in working order.  As I said, it was a winterly day; it had been snowing hard and the snow froze and accumulated on the upper edge of the signal arm until its weight bore it down.  That is a fact that no fiction writer dare have invented, but to this day every signal on the Great Northern pivots from the centre of the arm instead of from the end, in memory of that snowstorm.”