Snowy tree crickets chirp faster as the night warms. Turn the dial and the whole field speeds up — at first each caller keeps its own time, then the chorus falls into step. Count fifteen seconds of chirps, add forty. That is the temperature.
Reduced motion is on — the field is drawn still and the chirp rate is shown as a number.
Amos Dolbear published a one-page note observing that the snowy tree cricket's chirp rate rises in near-perfect proportion to air temperature. The insect is cold-blooded; the muscles that scrape its wings run faster when warm.
The relationship is linear enough to invert. Give the law a chirp count and it hands back a temperature — no thermometer, no mercury, just arithmetic and a quiet night. The field above runs the law forward: set a temperature, hear the rate it demands.
Early in the evening they drift — each cricket on its own clock, a scatter of overlapping chirps. Wait, and the field couples: the calls pull toward one another until the whole chorus pulses as one.
Once the chorus locks, every pulse is one countable chirp. Mark the start and count each pulse of the field for a slow, deliberate fifteen seconds.
Whatever number you reach, add 40. The count strip on the console does this live — it should land within a degree of the dial you set.
That sum is the air temperature in Fahrenheit. The original folk rule counts fourteen seconds; fifteen matches Dolbear's law exactly, which is the count this instrument keeps.