The post-road by which we had come was therefore impossible, and Kennedy swung up into the country, in the hope of throwing off pursuit long enough to give us a better chance.

“Take the wheel, Walter,” he muttered.  “I’ll tell you what turns to make.  We must get to the State line of New York without being stopped.  We can beat almost any car.  But that is not enough.  A telephone message ahead may stop us, unless we can keep from being seen.”

I took the wheel, and did not stop the car as Kennedy climbed over the seat.  In the back of the car, where Mrs. Cranston was sitting, he hastily adjusted the peculiar apparatus.

“Sounds at night are very hard to locate,” he explained.  “Up this side road, Walter; there is some one coming ahead of us.”

I turned and shot up the detour, stopping in the shadow of some trees, where we switched off every light and shut down the engine.  Kennedy continued to watch the instrument before him.

“What is it?” I whispered.

“A phonometer,” he replied.  “It was invented to measure the intensity of sound.  But it is much more valuable as an instrument that tells with precision from what direction a sound comes.  It needs only a small dry battery and can be carried around easily.  The sound enters the two horns of the phonometer, is focused at the neck, and strikes on a delicate diaphragm, behind which is a needle.  The diaphragm vibrates and the needle moves.  The louder the sound the greater the movement of this needle.

“At this end, where it looks as though I were sighting like a surveyor, I am gazing into a lens, with a tiny electric bulb close to my eye.  The light of this bulb is reflected in a mirror which is moved by the moving needle.  When the sound is loudest the two horns are at right angles to the direction whence it comes.  So it is only necessary to twist the phonometer about on its pivot until the sound is received most loudly in the horns and the band of light is greatest.  I know then that the horns are at right angles to the direction from which the sound proceeds, and that, as I lift my head, I am looking straight toward the source of the sound.  I can tell its direction to a few degrees.”

I looked through it myself to see how sound was visualized by light.

“Hush!” cautioned Kennedy.

Down on the main road we could see a car pass along slowly in the direction of Montrose, from which we had come.  Without the phonometer to warn us, it must inevitably have met us and blocked our escape over the road ahead.

That danger passed, on we sped.  Five minutes, I calculated, and we should cross the State line to New York and safety.

We had been going along nicely when, “Bang!” came a loud report back of us.

“Confound it!” muttered Kennedy; “a blowout always when you least expect it.”

We climbed out of the car and had the shoe off in short order.