COPY.
Appendix 111.
512
Datum.
taking the lowest tide at each new and full
moon, say 25 tides in a year, it is found that their average
level is about 4 inches above Ordnance Datum. This result seems
to convey most nearly the idea of a level equivalent to mean low
water of spring tides in the British Isles. If both day and night
spring tides were taken here, or 50 in a year, the resulting
mean level would be much higher, the daily inequality being so
great that two consecutive low tides are often really high and
low relatively to each other.