LUNAR TIDE ON LAKE MICHIGAN. Accompanying Memoir by Lieut. Colonel J.D. Graham, U.S.Topographical Engineers. Fig. 1. The average, or mean result of all the observations,/9184 in number), made half hourly at Chicago, between the 1 of January and the 1 of July 1859. TABLE 1. Showing the half-hourly (and in two places the quarter-hourly) co-ordinates of altitude of the average semi-diurnal lunar tidal wave at Chicago, on Lake Michigan, derived from 9184 observations made between January 1st and July 1st, 1859. The accompanying profile, marked Fig. 1, shows the mean semidiurnal tidal wave at Chicago, projected from the foregoing co-ordinates, embracing every vicissitude of winds and weather, &c., which occurred during the whole six months' observations. It shows the altitude of this mean tidal wave to be, at its summit, one hundred and forty-six thousandths (.146) of a foot, equal to 1 inches; and the average time of high-water is thirty minutes after the time of the moon's meridian transit. transit. On a close examination of all the observations embraced in the series, we find one hundred and eighty-nine (189) which we think ought to be rejected, because influenced in an extraordinary degree by unfavorable winds. This would reduce the number of observations in the series to 8995; and each co-ordinate of altitude would depend on a mean of three hundred and thirty-three observations, and stand as follows, viz. : TABLE 2. Showing the half-hourly (and in two places the quarter-hourly) coordinates of altitude of the average semi-diurnal lunar tidal wave, at Chicago, on Lake Michigan, as derived from 8995 observations, made between January 1, and July 1, 1859. After the moon's meridian Before the moon's meridian transit. |