Mr William Galbraith on the Tides and Dew-Point. 9 
sun and moon, and the distance of these bodies from the earth. They 
are, in fact, proportionally more considerable when the sun and moon 
are nearer the earth, and in the plane of its equator. It may be 
remarked that, in general, on our coasts, the highest tides follow the 
times of new and full moon by about a day and a half or thirty-six 
hours, except perhaps in some peculiar localities in rivers and deep 
bays or salt water lochs, intersected by shallows and narrows, in 
which there are several tides at the same time. 
The time of high water at new and full moon is therefore a very 
important element in the computation of the time of high water on 
a given day, and is generally called the establishment of the port 
when the moon is at her mean distance, or when her horizontal 
parallax is 57’ nearly. 
On observing attentively the height of the tides, which happen 
thirty-six hours after the syzygies at the equinoxes in any port, when 
the sun and moon are at their mean distances from the earth, we 
find, from a mean of all the heights, a certain quantity u, the unit 
of height, which varies with each port or place, and its application 
is shewn in page 398, &c., of my edition of Ainslie’s Surveying, 
published by Messrs Blackwood in 1849. 
I shall now proceed to shew the method of determining the value 
of u, the unit of height, and of the height of a given point above 
mean tide, though for want of the necessary apparatus of the best 
kind, and my limited time, at a period of the year when I could not 
have the sun and moon both on the equator, the final result will not 
be that required correctly. If the method indicated be followed 
nearly by others who are more favourably situated, considerable ad- 
vances may be made to greater precision. The requisite allowance 
for the effect of the height of the barometer and thermometer has 
never, in this country, to my knowledge, been at all applied, though 
when accuracy is required it is indispensable. The merit of the dis- 
covery of the constancy of the mean level of the sea has been re- 
cently claimed by the writer of a book on Marine Surveying, for his 
father, who made it about the year 1830. Why, the investigations 
of Laplace depend upon this constancy ; and it is stated annually in 
the Connaissance des Tempes, that the unit of height, u, at Brest, 
is known with great accuracy, because it was determined from a 
series of sixtcen years’ observations, from 1806 to 1823!!! 
To allow for the effect of the height of the barometer, it must be re- 
collected that the specific gravity of mercury at 30 inches of the baro- 
meter, and 50° of Fahrenheit’s thermometer, is 13-574, while that of 
sea-water varies, in different parts of the ocean, from about to 1:026 to 
13:574 
ints 
1:027 
nearly, the ratio of the specific gravity of mercury to that of sea- 
water, at the above pressure and temperature. Whence the formula 
for the effect of the pressure of the atmosphere, on the rise of the 
1-028, and 1-027 may be taken as the mean. Whence, 
