TIDE-LEVELS 1D 
of no importance, and that the total times of submergence and exposure 
each fortnight, or for the whole growing season, are the decisive factors limit- 
ing the distribution of a plant. There is, however, good evidence that the long 
exposure during an extreme low tide on a clear, hot day may result fatally to 
plants that have flourished during average tides, e. g., Ulva seems often to be 
killed off by long exposure and drying up, after the draining off of the water 
from the underlying mud. 
TIDE-LINES. 
After the standard tide-stake and the Reith stakes were established, the 
method of determining the position of the 5, 6, 7, and 8 foot contours, or 
tide-lines, on the beach was as follows: 
An ohaserer stationed near a tide-stake gave etna by a whistle or flag when 
the water had risen to within 1 inch of the level to be marked, another signal 
when the water reached the exact level, and a third when it reached 1 inch 
above this. One or more workers on the beach, with bundles of stakes, each 
marked with the height of the level that was being determined, began to set 
these at 50-foot intervals along the beach. On hearing the first whistle, stakes 
were set at an estimated height of 1 inch vertically above the water-line. At 
the second signal the stakes were set exactly at the water’s edge. At the time 
of the third signal they were set in water 1 inch deep. By knowing the rate of the 
rise of water at that stage of the tide, the stakes could be set nearer and nearer 
the water’s edge after the first signal, and deeper and deeper in the water as the 
third signal approached. Thus the exact position of the water-line was marked 
quite accurately by stakes for each foot from the 5-foot level to the 8-foot level. 
The position of each stake was then plotted on the map, and with map in hand 
in the field these points were connected up to form the contour-lines. 
The contours of the Marsh and Sand-spit, above 8 feet, were determined 
by a surveyor’s level. The —1-foot, 0-foot, +1-foot and +-1.5-foot contours 
were plotted by noting, when the water was at each of these levels, the point of 
intersection of the water-line with each of the longitudinal and transverse 
range-lines. The contours between —2 feet and —? feet were determined by 
soundings. 
The stakes marking the 6-foot, 7-foot, and 8-foot tide-lines were left on the 
beach throughout the season. By the aid of these it was possible accurately 
to determine the exact vertical distribution of the plants of beach and Marsh. 
The vertical distribution of plants on each wharf was determined by the 
graduated tide-stake beside it. 
