# Page:Climatic Cycles and Tree-Growth - 1919.djvu/112

<poem>﻿Theory. — The formula for the period is very simple: Let﻿ y = length of curve in years or other time-unit employed. ﻿l = length of curve image across sweep lines in centimeter or other unit of length. ﻿s = spacing center to center of analyzing lines in unit of length. Then﻿${\displaystyle {l \over s}}$ = number of analyzing lines in curve when lines are parallel to sweep. ﻿${\displaystyle {ys \over l}}$ = number of years in 1 line when lines are parallel to sweep.