Mr William Galbraith on the Tides and Dew-Point. 11 
From formula (2.) there will be found 
411 (6—30 in.) = +11 (31 in.—30 in.) = +1l1x1 
= +1°1 foot. 
Hence, 22°9 feet + 1:1 foot =24 feet, at 30 inches. 
In this manner, all registers of tides should be uniformly reduced, 
otherwise considerable irregularities would be unaccounted for. 
There is another irregularity to which the rise of the tides is sub- 
jected, and that is the force and direction of the wind, which, in some 
localities is very considerable, and is more difficult to be estimated 
correctly. But if tide and wind gauges were always combined with 
barometers, as they certainly ought to be, the effects of the baro- 
meter and force of the wind could be eliminated, and each effect ac- 
curately known and determined. 
I shall now shew the method of determining the height of the 
mean tide, or the mean level of the sea, as if there were no tide, and 
of referring a given point to it, from which point a series of levels 
may be carried over a country, and referred to the mean level of 
the sea. (See Ainslie’s Surveying, new edition, 1849, p. 397, &c., 
Broddick, Arran, 30th July 1849. 
Method of Determining the Height of the Mean Tide, and of referring 
a given point to it. 
Barometer 29-42 inches, Thermometer 64° F. 
D 
24 Divided 
23 Staff in 
22 Feet. 
First High Water 
t 
H 
H 
1 
i 
' 
' 
1 
1 
1 
| 
! 
' 
1 
' 
' 
' 
' 
1 
1 
1 
1 
' 
1 
' 
' 
' 
1 
~ 
a 
SES SONA Go nH 
BSS RABKS 
Seco: igh W . 
Divided oC nd High Water Sie See OSNews Sessa 8 
slag 9\] 85 feet Mean Tide 9 
eet. || ------------—----------- ---- p00 ---- == === 8 
7 7 
silt Intermediate “Fy Low Wate, 5 
4 4 
3 3 
2 2 
1 1 
(Vl cet EE Se 8 tg NE 2 Se re a eS ae aE et o 
BE 
If the observations be made at a pier or quay-wall, and the height of 
the tide can be read at both high and low water ; one divided gauge, 
as AB, will be sufficient. Also the height of the point p, may, by 
the usual spirit-level, be transferred to any rock or public building 
in the vicinity which may be convenient, and there permanently 
marked for future reference. 
