1841.1 



THE CIVIL ENGINEEU AND ARCHITECTS JOURNAL. 



151 



the level of the enginemau's eyes; and that the point where the cen- 

 tre of the beam would intersect the horizon, A C, of his vision at E, 

 should be about 700 feet from the lens. The impulse of the light 

 would be most advantageously received at surae point as near the lens 

 as is consistent with a full effect from a flame placed in its principal 

 focus. A more remote observer would receive the rays diluted by 

 distance : while a nearer approach of the eye to the lens would render 

 it necessary to adopt an ex-focal arrangement, so as to cause conver- 

 gence of the rays. By the latter iu-rangement their divergence would 

 be decreased, and the space covered by the light would be lessened 

 not only in proportion to the decrease of divergence, but also to that 

 of the cosine of the beam's inclination to the horizon. Both these 

 circumstances would therefore combine to curtail tlic duration of the 

 impression on the eye. 



It may naturally be expected that I should say something regarding 

 the duration of the impu'se of the light on the eye; and upon this 

 topic I shall, in absence of actual experiment, content myself with 

 Stating briefly the result of my calculations. If we suppose that an 

 effective divergence of only "2" were to be obtained (and this is just 

 one thiid of what is obtained from Fresnel's lens with the great lamp), 

 ! find that the light would spread itself along the horizon of the ob- 

 server's eye between B and C to the distance of about lOUO yards, 

 which, at the speed of 40 miles an hour, would be passed over in about 

 50 seconds, but at the ordinary railway speed of 25 miles an hour, 

 about SO seconds or I3 minute would be required. Such a flash of 

 light falling upon the polished parts of the engine, and upon the ob- 

 server's face, would undoubtedly act as a most effective signal. If, 

 however, it should be thought advisable to increase the duration of 

 the impression by spreading it over a greater length of the Hue, this 

 effect could be easily produced by a slight alteration of the inclination 

 of the lens, so as to cause the line of railway to cut the refracted beam 

 more obliquely ; but I by no means expect that any such modification 

 would be found necessary in practice. The nearness of the eye to the 

 lens, and the brilliancy of the flash, would, I am inclined to think, more 

 than comjiensate for the shortness of the impression. 



I must add a few words regarding the expense of these siguals, 

 which would be made up of the cost of erecting the scaffold of car- 

 pentry, the price of the lens, and the maintenance of the light. The 

 price of the stage I shall pass over as a matter which may vary ac- 

 cording to the circumstances of the situation and the taste of indi- 

 viduals ; but the cost of the gieat annular lens does not exceed 40/.; 

 and if a smaller sized lens, which I think would be found quite suffi- 

 cient for the purpose, were employed, the expense would not be more 

 than 10/. The annual maintenance would consist of little more than 

 the supply of a gas or an oil burner. The consideration of the ex- 

 pense, therefore, of maintaining such a system of signals at the ne- 

 cessary intervals on railways, is not for a moment to be set against the 

 most remote risk of the least of all the numerous accidents, the records 

 of which fill the public prints. 



OBSERVATIONS (3N THE MOTIOX.S OF SHINGLE BEACHES. 



By Hejjrs- R. Palmer, EsvJ., F.R.S.* 



Fiotii llie Philosophica! Tfansactions o/the Royal Society: — read 

 April 10, 1834. 



The extraordinary prevalence of tempestuous weather during the 

 last autumn having occasioned numerous disasters on our coast, the 

 public attention was directed in an unusual degree to the imperfections 

 of many of the harbours, and more particularly to those which are 

 encumbered with accumulations of shingle. The access to harbours 

 thus circumstanced is generally uncertain, and in tempestuous weather 

 is frequently dangerous, or even impossible. 



The action of the sea, which gives motion to the shingles and pro- 

 duces the evils complained of, has long been a subject of speculation ; 

 but I have not found that it has been systematically investigated. 

 Indeed, the contrariety of opinions advanced upon the subject, suffi- 

 ciently indicates an entire absence of that satisfactory mode of inquiry 

 which is essential to the foundation of a safe and practical deduction. 



Very little has been written upon the subject ; and such facts as 

 have been mentioned have only been referred to incidentally, or with 

 a view to geological science. My present object is exclusively prac- 

 tical in its nature, and my observations have been limited to such facts 

 as would assist in establishing certain and fixed rules for controlling 



' The cunstruciion of harbours, piers, and breakwaters is like'y :o become 

 of considerable iniix)rtance to the engintering proU'ssion ; we therefore pro- 

 pose lo collict for publication in the Journal, such papers as have been 

 wrillen on tli» subject. 



the motions of the beacli, so far as to enable us to preserve a clear 

 channel through it in all seasons, and in every variety of weather; and 

 to accumulate and preserve the shingles, where it is needful to do so. 



The subject at first sight appears greatly complicated ; and were it 

 necessary to discuss minutely all the modifications arising from the 

 variety of forms and local circumstances, it would perhaps be too 

 much so for general description. I have, however, limited my inves- 

 tig atin to those simple and unvarying laws to which natur.- always 

 adheres; and therefore the following observations must be considered 

 as restricted only to certain general principles, subject to a variety of 

 modifications. 



The principles which I propose to illustrate will (under similar cir- 

 cumstances) at all times exhibit the same phenomena, but for the sake 

 of perspicuity I shall now only refer to the coasts of Kent and Sussex. 



Section 1. 



That the pebbles which compose the shingle beaches on these 

 coasts are kept in continual motion bv the action of the sea, and that 

 their ultimate progress is in an easterly direction, are facts long known 

 and commonly observed. The following observations are chiefly di- 

 rected to the particular manner in which the motions are produced. 



From a general view of the effects that I have noticed, it appears 

 that the actions of the sea upon the loose pebbles are of three kinds: 

 the first heaps up, or accumulates the pebbles against the shore ; the 

 second disturbs, or breaks down the accumulations previously made ; 

 and the third removes, or carries forward the pebbles in a horizontal 

 direction. 



For convenience I propose to distinguish these by the following 

 terms, viz. the fir^t, the accumulative action; the second, the destruc- 

 tive action; the third, the progressive action. 



All the consequences resulting from these various actions are ex- 

 clusively referrible to two causes. The one is to the current, or the 

 motion of the general body of the water in the ebbing and flowing of 

 the tides; the other to the waves, or that undidating motion given to 

 the water by the action of the winds upon it ; and it is of considerable 

 importance to the present inquiry that the effects resulting from each 

 specific cause be separately considered. 



The motion of the shingles along the shore is commonly attributed 

 to the currents, the action of the waves being considered only as a 

 disturbing foace. That such a notion is erroneous will, I apprehend, 

 presently appear; although I have to regret that I have not had the 

 opportunity of obtaining such satisfactory information relating to the 

 velocities of the currents in the cliannci, as would have enabled me to 

 include every form of argument upon the subject. The absence of 

 such information has also prevented me from deciding satisfactorily 

 as to the sources from whence the whole body of shingle is derived, 

 which, although not necessary for the practical purposes I have ia 

 view, would have given more interest to tbe subject, and would hive 

 rendered the elucidation more comjjlete. I must, therefore, for the 

 present, be content to pursue the motions of the beach after it is found 

 lying along or near the shore ; observing only that the materials of 

 which it is composed are those of the various strata in the vicinity of 

 the coasts, together with the ordinary sea sand, and such small parti- 

 cles as may have been brought to the shore by the floods of the various 

 rivers. 



That the current is not the force which moves the pebbles idong the 

 coast, will appear from the following reasons ; 



1st. If it were so, the direction of the motion of the pebbles would 

 be deterniined by th it of the currents ; but while the direction of the 

 currents will vary with the changes of the tides, we find that the di- 

 rection of the pebbles may remain unaltered ; and also that the motion 

 of the pebbles is continued where no current exists. 



•2nd. Although the velocities of the currents may not have been 

 ascertained with precision, yet it is known that the velocities generally 

 along this coast, which can possibly act on the shingles, are not suffi- 

 cient to give motion to pebbles of every dimension, which are in fact 

 carried forward. 



3rd. The motion of a current will not produce that order in which 

 the pebbles are found to lie, which order (as will be hereafter shown) 

 may easily be distinguished as the effect of the motion of the waves 

 only. 



Irhe direction of the waves is determined principally by the wind, 

 the prevailing direction of w hich on the coasts referred to is from the 

 w eslward. E\ery breaker is seen to drive before it the loose materials 

 which it meets; "these are thrown up the inclined plane on which they 

 rest, and in a direction corresponding generally with that of the 

 breaker. In all cases we observe that the finer particles descend the 

 whole distance with the returning breaker, unless accidentally de- 

 posited in some interstices ; but we perceive that the larger pebbles 

 return only a part of the distance ; and upon further inspection we 



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