12 Mr William Galbraith on the Tides and Den- Point. 
In the diagram, A B is the divided staff, like the common levelling 
staff, into feet, tenths, &c., fixed on the shore, near high water mark 
in spring tides; CO, another, near low water mark, or such other 
convenient position as may be thought necessary, with such others, 
intermediate if need be, and SHO, the sloping shore or beach. 
Then the first high water, marked A in the diagram and formule of 
reduction in Ainslie, rises on the staff A B to 13 feet, the next ob- 
servation or intermediate low water, marked H, falls to 5 feet on 
the staff C O, and the next or second high water rises to 11 feet on 
the staff AB. From these it is required to find the mean height 
of the level of the sea, 8-5 feet on the staff A B, or 12°5 on DE, 
the height of the given point p, above the mean tide, the barometer 
being at 29°42 inches, and the barometer at 64° Fahrenheit. 
Likewise the height of the point p above mean tide, when the baro- 
meter stands at 30 inches, and Fahrenheit’s thermometer at 50°. 
Barometer observed, . 29:42 inches. Thermometer 64°. 
Reduction from 64° to 50° — 04 
Barometer ‘ - . = 29°38 inches at 50° Fahr. 
Whence by the formula, Ainslie, page 397, 
h=13 feet on the seale of the divided staff. 
h’=11, and 
2H=10 
4)34 
Height of mean tide=8°5 feet above zero of the scale A B. 
Height of p=21°0 feet above zero. 
Height of p=12°5 feet above mean tide on D E. 
By formula (2.) +11 (29:38—30)= + 1-1 x —0°62= — 0682 
feet. 
Hence 8°500 feet — 0-682 feet= 7-818 feet on AB, the mean height 
of the tide reduced for the barometer and thermometer, and 12-500 
feet + 0°682= 13-182 feet, the true height of p above the reduced 
mean tide. 
In this manner both the position of the mean level of the sea and 
the height of a given point above it, may be readily obtained, due 
allowance being made for the state of the barometer and thermo- 
meter. 
It occasionally happens that an observer cannot command the 
assistance of the best apparatus to conduct the observations in the 
preceding manner, as was the case with myself at Broddick this 
year; yet I was anxious then, as well as on former occasions, to 
determine the position of the mean level of the sea as nearly correct 
as possible, in the circumstances in which I was placed. For this 
purpose I selected the upper surface of Broddick Quay as my point 
p, and on a kind of rude, inclined jetty, at right angles to the sea- 
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