TY  - RPRT
TI  - Letter Circular 600: timekeeping through the ages
AU  - Vandoren, Katharine S
PY  - 1940
PB  - National Institute of Standards and Technology
CY  - Gaithersburg, MD
SN  - LCIRC 600
DO  - 10.6028/NBS.LCIRC.600
ER  -