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NATURE 
| Fan. 4, 1883 
the detriment of therest. The first chapter gives a brief 
sketch of Chinese history, the second of the system of 
administration; various chapters are then devoted to 
popular customs, to education, medicine, music, dress 
and food, architecture, honours, names, superstitions, 
religions, &c. There is also an excellent map. To the 
general reader who desires some accurate information 
respecting a country which is coming nearer to us every 
day, or to the student who wants a vade mecum, no better 
volume can be recommended. 
LETTERS TO THE EDITOR 
[The Editor does not hold himself responsible for opinions expressed 
by his correspondents. Nether can he undertake to return, 
or to correspond with the writers of, rejected manuscripts. 
No notice ts taken of anonymous communications, 
[The Editor urgently requests correspondents to keep their letters 
as short as possible. The pressure on his space ts so great 
that it is impossible otherwise to ensure the appearance even 
of communications containing interesting and novel facts.| 
On the Occurrence of Great Tides since the Commence- 
ment of the Geological Epoch 
Mr. BALL says very truly that the fundamental question is, 
what traces of great tides ought we to expect to find if those 
great tides had really existed? Mr. Darwin says, coarse-grained 
rocks, and different forms of vegetation calculated to resist the 
action of the accompanying great winds. Mr. Ball, in reply, 
remarks that high tides in the Avon are accompanied by fine | 
sediment. He thinks with others that the high tides would 
have produced a vastly greater amount of sediment than is being 
formed at present. I quite agree with Mr. Ball about the fine 
sediment, but I am not at all clear that high tides mean great 
marine denudation. By far the largest portion of the work 
done by the sea as a denuding agent is due not to the wearing 
action of currents, or to the pounding of materials on a beach at 
low water, but to the direct action of the sea on the cliffs. This 
force is estimated as about a ton per square foot on the average 
in winter, on the west coast of Great Britain. This undermines 
the cliff at the sea level, and then the top part falls partly by its 
own weight, but still more through the effect of the air com- 
pressed in the caves and cracks, which by its elasticity spread 
the blow over a very large surface through the crack and joints 
of the rock. Now to undermine cliffs with a given force of 
wind and wave, it seems clear that the maximum effect would be 
produced where the tides are very small, for there the force is 
constantly applied at the same spot. Witha rise and fail of 
100 feet, each portion of the cliff would be subjected to the force 
of the waves for so short a time that in all probability caves 
would never be formed at ali, and the height of the tide would 
be an actual protection to the land. Asa matter of fact, those 
places where the tides are highest show, as far as I know, no 
signs of excessive denudation. I spent two days last year on 
the Bay of Fundy, where the tides are bigher than anywhere in 
the world, and I was very much struck with the absence of any 
evidence of great denudation due to the tide. The cliffs at the 
Loggins are about high-water mark, with a long beach which | 
slopes very gradually, The force of the waves, such as they 
are, is spent in hammering this beach and grinding it into fine 
sand and mud; the mud is carried about in suspension by the 
tide, and the sand is shifted about, but the denuding effect is 
exceedingly small. The consequence is that the cliffs are 
pounded by the waves for such a short time each tide that they 
suffer mainly from atmospheric denudation, the sea doing little 
more than keeping their base clear, and in many places not even 
doing that. 
Similar phenomena are presented by the hizhest tides in Great 
Britain—tho-e on the Severn. 
the tide rises 30 feet; there are no waves; the banks are 
covered with a thick coating o° mud and denudation is nil. 
At Aust Cliff, again, on the Severn, where the soft red marls are 
peculiarly liable to erosion, the height of the tide is again a pro- 
tection. The cliff is about high-water mark, and the force of the 
waves is expended on the beach. The case is the same at Watchet, 
and a good many other places o1 the Severn. Ido not know 
of any part of the Severn remarkable for excessive denudation, 
owing to the high tides. There is a strong resemblance between 
the Bay of Fuudy and the Severn : there are the cliffs at high- 
Here at Clifton on the Avon | 
water mark, the same long beaches, the same shifting sands and 
mud in suspension ; similar causes have produced similar results. 
In narrow inlets like the Bay of Fundy and the estuary of the 
Severn, these high tides mean rapid currents and small waves; 
but along shores freely exposed t> the ocean, the highest tides 
might be accompanied by very feeble currents, But if, as Mr. 
Darwin says, the high tides were acrompanied by trade winds 
about 3} times as strong as the present ones, the battering power 
of the waves and the strength of the currents would be very 
greatly increased, and plains of marine denudation, it might be 
supposed, would be very rapidly formed. What would be likely 
to happen if such winds and waves began now to act on our shores ? 
Should we have reason to expect that England would in a com- 
paratively short time disappear beneath the waves? A very rapid 
destruction of our present cliffs would undoubtedly begin, though, 
as I pointed out before, this would be attributable mainly to the 
winl, and not to the tide; the cliffs would be driven back to 
about the ordinary high water mark, leaving a long shelving 
beach extending to a few feet below the low water mark. The 
cliffs would then for far the greater portion of each tide be entirely 
free from marine denudation, and their rate of wasting would 
depend on the power of the sea to tear up the long solid sloping 
beach, and restore comparatively deep water at the base of the 
cliffs. But this process is an exceedingly slow one, because 
there can be no undermining or assistance from compressed air, 
and I should anticipate that marine denudation would then be 
actually slower than it is at present. There would, of course, 
be abundance of very fine sediment formed during the first 
wearing back of the cliffs by the grinding of the materials 
between tide marks ; but when cnce the cliffs had reached the 
high water line, the amount of sediment would depend chiefly on 
the amount of atmospheric denudation, supposing that the sea 
kept the base of the cliffs clear. We should, in fact, have a 
repetition of the phenomena presented to us now by the Bay of 
Fundy and the Severn. Thus far, then, it seems to me that no 
argument can be drawn from the fineness of the early sediments 
against the existence of high tides in the Geologic period ; nor, 
on the other hand, does the quantity of seliment seem to me a 
strong argument in favour of it. But Mr Darwin’s argument 
that the vegetation of the Carboniferous period could not possibly 
have held out against the violent winds which necessarily accom- 
panied these high tides seems to me unanswerable. One has 
only to reflect on the effect produced by our present winds to 
feel convinced that if the winds and tides went together they 
were certainly Pre-Carboniferous, and almost equally certainly 
Pre-Devonian. J. G. GRENFELL 
Clifton College, December 29, 1882 
Sir George Airy on the Forth Bridge 
As Sir George Airy’s last letter may, like his first, provoke 
replies from di-tinguished American and continental engineers, 
it may save your correspondents’ time and your own valuable 
space if I add a few final words in explanation. 
1. Sir George says :—‘‘ The danger of buckling in a hori- 
zontal direction with a length of 340 feet, remains undiminished 
unless it is counteracted by bracing unknown to me.” Now 
Sir George evidently has forgotten that some time ago he was 
furnished with photogra, hs of a larse model of the bridge taken 
with the view of showing the said bracing, and that his attention 
was specially directed to the point. 
2. Sir George thinks ‘‘it desirable that attention should be 
called to the magnitude of the forces concerned,” and speaks of 
a wind-pressure of 75 tons, and an end-pressure of 600 tons. 
Now he clearly has forgotten that before he wrote his first letter 
a ‘‘stress diagram” was sent to him, on which it was noted 
that the wind-pres ure provided for was 2207 tons on each 
span, and that the estimated end-pressure on the strut referred 
to was 2380 tons. 
3. Sir George holds the engraver responsible for some of the 
alarmist statements in his first letter. I must remark, therefore, 
that it was pointed out that the bridge would have been perfectly 
safe had the details of the desizn been as he assumed. For 
evidence that a 340 feet tubular strut of 12 feet diameter 
would not fail in the manner stated by him, he was referred 
to Hodgkinson’s experiments as published in the PAdosophi- 
cal Tyansactions, and Clark’s work on the Britannia bridge; and 
further, he was lent the Z7amsactions of the American Society 
of Civil Engineers for last year, containing the most recent 
experiments on long wrought iron columns. Any or all of 
the-e docunents would have shown him‘ that the Forth bridge 
