110 
CAPTAIN F. W. BEECHEY ON THE TIDES IN 
Table for the 
reduction of 
soundings. 
Unequal mo- 
tion of the 
upper and 
lower half of 
the tide- 
wave, com- 
mon to all 
the tides in 
Irish Sea and 
Bristol 
Channel. 
Apparent 
mean eleva- 
tion of the 
water. 
placed over the hook of a small hand steelyard, and the line brought tight until the 
index noted a given quantity, which was afterwards considered the standard ; and 
by comparing the observations of several days obtained in this manner with the tide 
gauges on shore, a near agreement satisfied me that the results were to be depended 
upon. At the same time it is not pretended that the rise has been determined to any 
great accuracy, although I think it is within the foot, and quite near enough for the 
purposes of navigation for which the chart is intended. 
To make this chart useful for the reduction of soundings at intermediate times be- 
tween high and low water, I have annexed a diagram, Plate VII., constructed from a 
vast number of observations, and have given directions for using it. 
I shall now notice some peculiarities in the tides which have fallen under our ob- 
servation. In the construction of the Table, Plate VII., for the use of the seaman, in 
the Irish Sea it was found that the. place of the water at the half -tide interval did not 
correspond with that of a mark at the half range of the wave, but that it was always 
below it, showing that the upper half of the wave rose and fell more rapidly than the 
lower. It was also found that the curve of the Irish Sea tide did not correspond with 
that of the Bristol Channel tide ; that neither followed the law of the sines to cor- 
responding arcs of tidal intervals, upon the falling water especially, as may be seen 
in Plate VIII., constructed for the purpose of showing the place of the water at cor- 
responding intervals of a falling tide at places distantly situated. 
It was also seen that, owing to the unsymmetrical form of the curves, it would be 
necessary to have tables for the reduction of soundings adapted to rising and falling 
tides respectively. In Plate VI. I have exhibited the discordance of the curves of the 
rising and falling wave in the Bristol Channel, where it will be seen that at one stage 
of the tide, the difference of similar tidal intervals amounts to about four feet. 
I may here mention that the Cumberland Basin Gauge (Mr. Bunt’s), which has 
been frequently referred to in the Philosophical Transactions, corresponds very nearly 
with the Portishead Gauge with which the observations in Plate VI. were made. 
In connection with the range of tide is that of the apparent mean elevation of the 
water. All the observations confirm the remark of Professor Airy*, viz. that this 
mean level is higher at the springs than at the neaps. The mean place of the water 
however for an entire lunation, during the summer months at least, is tolerably con- 
stant, and affords a fair standard to which the reductions used in our nautical surveys 
may be referred in the event of the gauge being removed by which the observations 
were made. 
As an example, I annex the result of observations made at Holyhead during nearly 
four entire years. 
* Philosophical Transactions, 1845, Part I. p. 31. 
