covered Langley's salary and the bills. Allegheny Observatory continued to supply time signals until the US Naval Observatory started offering it for free in 1920.

Langley's 1879 invention of the Bolometer

In 1878, Langley invented the bolometer, which makes very sensitive detections of thermal radiation. He used it to measure the strong variation with wavelength of Earth's atmospheric absorption, results used by Arrhenius in 1890 demonstrating the

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