First Edition. 8vo, paged 174-177, illustrated; G+; brown cover with black print, bumped corners throughout, creasing to right edge and all corners, slight smudging; back has blue library stamp at right, creasing, slight smudging; clean interior; "Reprinted from the Review of Scientific Instruments, Vol. 7, No. 4, pp. 174-177, April, 1936"; "Papers from the Geophysical Laboratory, Carnegie Institution of Washington, No. 897."
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