10 Mr William Galbraith on the Tides and Dew- Point. 
tide, assuming 30 inches for the mean standard height at the level 
of the sea, will be 
— 13-2 (b—30 in.) in inches ; or 
=. 1:1(6—30.in.):in feeticncay © aay), (1.) 
Since it is the more convenient method to record the rise and fall 
of the tide in feet and decimals, the last form will be the more ap- 
propriate, though the former is that given in Ainslie’s Surveying. 
To shew the use of this formula, we shall give one or two ex- 
amples. 
1. Suppose the tide, when the barometer stands at 30 inches, to 
be 24 feet, what would it be if the barometer stood at 28 inches 2 
Here, —1:1(6—30)= —1+1 x (28—30)= —1-1 x —2= +422 
feet. 
Hence, 24 + 2:°2= 26:2 feet, the real height. 
2. Suppose the tide under 30 inches rises 24 feet as before, what 
would it rise under 31 inches 2 
Here, —1+1 (6>—80)=—1:1x 1=~—1°'l feet. 
Hence, 24-0 —1:1=22°9, the actual rise. 
The difference of these two, or 26:°2—22-9=3°3 feet. Hence 
the difference of the predicted heights given in our almanacs must 
be liable to an uncertainty in this case of 3-8 feet, independent of 
the effects of the wind ; and this should always be allowed for when 
the exact height is of importance. 
When the rise of the tide is observed under a given barometric 
pressure, and it is required to reduce it to the standard,* which we 
have assumed at 30 English inches, the correction from formula 
(1.) must be applied with a contrary sign, that is, it becomes 
+1:1 (6—380 in.) ipl! ‘Sos. lestdet te eae 
3. Suppose the barometer stood at 28 inches when the tide rose 
26°2 feet, what would be the rise at the standard pressure of 30 
inches 2 
From formula (2.) we have 
+1:1 (6—30 in.) = +1:1 x (28—30) = 4+1:1x —2=-—2-2 
feet. 
Hence, 26:2 feet — 2-2 feet = 24-0 feet at 30 inches, 
4. Suppose the barometer stood at 31 inches when the tide rose 
22-9 feet, what would be the rise at the standard pressure of 30 
inches ? 
* In the valuable article on the Tides, in the Admiralty Manual, by Dr 
Whewell, Master of Trinity College, Cambridge, edited by Sir John F. W. 
Herschel, he says, p. 123, article 28, that “ 2, of an inch of mercury is equiva- 
lent to 1 inch of salt water,” and the coefficients would, instead of 1:1, be 1°67, or 
about one-half too great, while he gives no standard to which they ought to 
be reduced, 
