TIDE-LEVELS 13 
lowest stage of the tide was expected to reach exactly to mean low water. The 
correctness of this standard was checked later by other observations of the 
tide-level at both low and high water, and also, on quiet days, by comparing the 
level recorded on our stake with that predicted by the Tide Tables. This stake 
was graduated to feet and tenths of a foot for 2 feet below and 9 feet above 
the zero-point. 
When this standard tide-stake was once established, other similar graduated 
stakes were placed by the wharves on the east and west sides of the harbor, 
where records of the rise and fall of the water could be more readily made.” 
Stakes were also placed near the dense growths of alg and other plants whose 
distribution was to be studied. A permanent reference level or benchmark 
was cut into a stone on the northeast corner of the foundation of the Biologicai 
Laboratory, at 14 feet above mean low water. Records of rise and fall of the 
tides were made by noting the time the water reached each foot and half-foot 
mark on the tide-stake. Just before and after low water and high water the 
record was made for each tenth of a foot. This gives a more satisfactory 
record than that obtained by noting the height of the water at each interval 
of 10 or 15 minutes, since the contour-lines between tide-marks on the beach 
and marsh were determined for each half-foot interval. The records of the 
rise and fall of tide at the standard tide-stake were usually begun at or just 
before low water, and were continued through the rise to high water and 
through the succeeding fall to the next low water. In some cases the record 
was continued for 22 or 24 hours, in order to get the daily variations or 
differences in the height of low water and of high water in the two successive 
tides of each day. 
In the tide-curves here given, distances along the vertical axis indicate the 
height of the water above mean low water, in tenths of a foot, except at the 
middle and straighter part of the curve. Horizontal distances indicate time 
in intervals of tenths of an hour. The curve published in plate v1 shows a 
rather slowly ascending rise, taking nearly 2 hours, from low water at —0.2 
foot to +1 foot; a steep, nearly straight curve, taking 2.5 or 3 hours, from 
1 foot up to 6 feet; a very slow and irregular rise, taking about 2 hours, from 
6 feet to 7 or 7.5 feet. On the descent there is a slow and somewhat irregular 
fall from 7 to 6 feet or from 7.5 to 6.5 feet, lasting about an hour and a half; 
then follows a rapid descent to 2 feet, taking but 3 hours, and finally a slower 
fall from 2 feet to 0.2 foot in the morning tide or to 0.5 foot in the afternoon 
tide. This fall of 1.5 or 2.2 feet takes 2.5 hours, or nearly as long as is required 
for the drop of the 4 or 4.5 feet next above. The flattening of the curve on 
this side of low water is even more marked than that shown on the rise. 
In a confined harbor with a narrow inlet and with a surface area that may 
change very considerably with a slight change in level, it is to be expected that 
the tide-curve will show marked irregularities. As a matter of fact, the whole 
rise up to 6.5 feet gives a very regular curve. As one watches the rise of the 
water, however, between 6 and 7.5 or 8 feet, he sees the water remain almost 
constant at one level for many seconds or even two or more minutes, and then, 
suddenly, in a minute or less, go up a tenth of a foot. The record for the 
night tide, given in the curve on plate v1, shows this sort of irregularity very 
* See footnote on p. 133. 
