Tom Slade at Bear Mountain by Percy Keese Fitzhugh

Now something happened, trivial and barely perceptible, but it relieved the frightful situation of the moment. The flashlight flickered; it was less than a flicker; a scarcely visible flutter of the light for the fraction of a second. But the alert reptile projected its head like lightning. Its appalling rattle sounded in the hollow shaft; it hissed and its tongue darted.

Brent Gaylong knew what that suggestion of a flicker meant; he had tinkered often enough with the defective connection in that troublesome flashlight. Should he cast the stone down on the horrible thing's head? Suppose he should miss? Again the light flickered. Fortunately, that hideous, flat head was in the circle of brightness cast by the flashlight. It was poised menacingly a few inches above a flat bit of rock.