It thus appears, that the greatest effect of the sea, which has been observed, is that of the Atlantic at Skerryvore, which is nearly equal to three tons per square foot.

These experiments, amounting to 267 in number, and on the Atlantic alone, extending over 23 months continuously, are not intended to prove anything farther than the simple fact, that the sea has been known to exert a force equivalent to a pressure of three tons per square foot, however much more.

It is proper, however, to observe, that there may be some objection to referring the action of the sea to a statical value. Although the instrument might perhaps be made capable of giving a dynamical result, it was considered unnecessary, in these preliminary experiments, to do anything more than represent the maximum pressure registered by the spring, because the effects of the waves may, from supposing them to have continuity of action, be perhaps regarded as similar to a statical pressure, rather than to the impact of a hard body. The near coincidence, or indeed almost perfect agreement of the results of the experiments made with different instruments, goes far to shew that the waves act in very much the same manner as a pressure, although both pressure and impact must obviously enter into their effect. In the experiments, begun February 1844, and given at the end of the paper, the three instruments had not only different areas of discs, but very different powers of springs, and yet the results were almost identical. Now, the same force, supposing the waves to act like the impact of a hard body, would, in the Marine Dynamometer, have assumed very different statical values, according to the spaces in which that force was expended or developed; so that with the same force of impact, the indication of a weak spring would be less than that of a stronger.

In future experiments it may be interesting, however, to test the springs dynamically, by means of the impact of a heavy body dropped from a given height upon the plate or disc of the instrument. In some experiments lately made in this way, by dropping a cannon-ball upon the disc, it appeared, that, within the limits of the experiments, there was for each individual spring a ratio between the value registered by the leathern index and the calculated momentum of the impinging body. These ratios were, of course, found to vary in springs of different power, and to be constant only for springs of the same power. Did the waves, therefore, act by a sudden finite impact, like the cannon-ball employed in this instance, we could scarcely have found such harmony between the results of instruments with different springs, as the experiments alluded to afford. At the same time, the result cannot, perhaps, be in strictness considered correct; but, from the elongation of the spring being very small, the results may be regarded as practically correct,—the more so when we find so remarkable a coincidence of results as that alluded to.

Experiments.—With reference to the following experiments I have only to observe, that those which were made at Little Ross, upon the Irish Sea, cannot, from the unusual fineness of the weather at the time, be regarded as affording a true value of the effects of a hard gale in these seas. Of the others it is to be noticed, that where two or three instruments were for some time employed as a check upon each other, and only one or two readings are given, the want has occurred either from the instruments being under repair, or being difficult of access in stormy weather, or during neap tides. It often happened also, in consequence of the springs proving too weak, when new ones had to be made, or the area of the disc reduced. Registers of the state of the weather, apparent height of spray, &c., were generally kept; but it was not considered necessary to complicate the Tables by inserting these, excepting in one or two instances.

Dates.lbs. to a Square Foot.
Observations at Little Ross.
1842.
April25   15  
28   51  
June1   36  
4   81·5
20   86·5
24  840·0
25  458·0
July25  380·0
Aug.2  570·0
5  665·0
6  380·0
9  380·0
The Observations at the Skerryvore Rock, and the neighbouring Island of Tyree,distant 13 miles from the Skerryvore, are as follows:—
1843.
April24  455  
25  707  
May7  243  
11  182  
12  243  
16  364  
20-  495  
 476  
June3  182  
4  519  
7  428  
8  855  
9  173  
July2  476  
3-  723  
 866  
30  433  
Aug.9  346  
22  723  
30  389  
Sept.5  866  
21  952  
Oct.5 1535  
6 1606  
Nov.18 1711  
19 1497  
27 1497  
29 2353  
Dec.5 2674  
8- 3421  
At least
14 2460  
26 1947  
In January, two instruments were placed beside each other, but not set parallel.These instruments had springs of different power, the one being about double that of the other, and one had a discof 3 inches diameter, the other 6 inches.
1844.
Jan.6  962  
 928  
7 2353  
 357  
9 1925  
1000  
10  826  
1000  
Both instruments set parallel.
16  424  
 427  
Another instrument was placed beside them, but the two marked thus* were found tobe too weak, as the leathers were found flattened, and one of the instruments was broken, and was not repaired till the 15th February.
28 3422  *
2285  *
3313  
Feb.2  429  
 457  
3  429  
 457  
13  214  
 228  
15  321  
 280  
 321  
16  428  
 402  
 343  
24 1284  
1364  
 685  
26 2032  
2068  
 399  
27  321  
 321  
 342  
March4 3316  
3369  
3427  
7 1069  
 963  
 913  
10 1925  
1925  
1713  
11  535  
 481  
 456  
12 3316  
4011  
2970  
13 1142  
1283  
1283  
April10  457  
 428  
 481  
11  800  
12  343  
 321  
14  571  
 535  
16  571  
 642  
 481  
17  800  
 856  
 862  
18  571  
 481  
19  800  
 535  
 481  
22  913  
 482  
 962  
24 1942  
1604  
1370  
25 1283  
 343  
 321  
27  457  
 481  
- Night800
tide
 642  
30  229  
 241  
May15  343  
14  481  
June6  571  
15 1828  
July11 1028  
13  457  
18  914  
23 1532  
25  571  
26  971  
27  457  
28 1142  
29  286  
30  914  
31 1028  
Aug.1  571  
7  914  
13  914  
14  914  
21  800  
30 1713  
Sept.12 1028  
14  914  
20  457  
23 1142  
25  685  
30 1599  
Oct.2 2399  
3 1485  
4 1828  
11 3427  [88]
14 1599  
19 1599  
20 2513  
22  800  
24 1827  
28 1485  
29  457  
Nov.2 1942  
10 1028  
14 1257  
15 2056  
16 2056  
22 2627  
23 3427  
27 3199  
28 4112  
Dec.7 1369  
9 2738  
10 1825  
13 1925  
14 1027  
15 1764  

[88] On this occasion, 14 stones were slightly moved, and 14 scattered, all of which had been built into the round-head or end of Hynish Pier, which was still in an unfinished state, and a Dynamometer which was attached to the Pier, registered on this occasion 2557 lb. These stones weighed from 1 to 1¹⁄₂ tons, and exposed, when built into the wall, about two square feet of surface. The stone to which the instrument was fixed was turned upside down, although it weighed about 1¹⁄₄ ton = 2800 lb.

A more exposed point of the Skerryvore Rock was at this time chosen for experiment; and with the view of ascertaining the effect of the waves at different heights upon the rock, two instruments were fixed, the one (No. I.) several feet lower, and about 40 feet seaward of the other (No. II.). It was observed, that about half-flood the force of the waves was a good deal expended before they reached the place where No. I. was placed, from there being so little water on the rocks outside; whereas when the tide was higher, the waves were, from the greater depth of water, not so much broken when they reached No. II. The results of the Marine Dynamometer shew generally about twice the force at No. II. as at No. I.; a result which shews how important it would be to ascertain the relative forces of the waves at different levels upon our breakwaters and other seaworks.

Date.Remarks.No. of
Instru-
ment.		Pressure
in lbs.
per
Square Foot.
1845.
Jan.7Heavy sea.I.1714
...II.4182
12Very heavy swell.I.2856
...II.5032
16Heavy ground swell.I.2856
...II.4752
22A good deal of sea.I.2856
...II.5323
28Heavy ground swell.I.2627
...II.4562
Feb.5Fresh gales.I. 856
...II.3042
21...I.1827
...II.3422
24Fresh breezes.I.1256
...II.3802
March9Ground swell.I.1256
Waves supposed about 10 feet high.II.3041
11Short sea.I.1028
24Heavy sea.I.2281
Waves supposed about 20 feet high.II.4562
26Swell.I.1256
Waves about 6 feet high.II.3041
29Strong gale, with heavy sea, the highest waves supposed 20 feet high, and the spray rose about 70 feet.I.2856
...II.6083

Register of Observations on the force of the Sea, made at the Bell Rock, German Ocean.