154 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[May, 



The entrance of ttie harbour being much encumbered with shingle, an 

 additional )>ier or jetty was erected, and extended about two hundred 

 feet further into the sea without having approached the effect intend- 

 ed. It is true that some advantage was derived from the extended 

 pier, by incre;ising the distance between the most violent action of the 

 breakers and the still water of the harbour. The shingles, therefore, 

 pass the mouth in a more dispersed form than they originally did, and 

 hence they do not so readily form a barrier, neither does its perpendi- 

 cular height become so great. 



Much valuable information on this part of the subject is recorded in 

 Lyon's History of Dover, which, as it may at any time be consulted, 

 is not repeated here. I shall only remark, that from the succession 

 of experiments made at that place, the general result has been in a 

 considerable acquisition of new land, which, although valuable in it- 

 self, is not the object intended to be obtained. 



If, then, it be admitted that projecting piers will not prevent the 

 encumbrance about the mouth of a harbour, situated as those referred 

 to in the tract of the restless beach, it remains to be seen how far such 

 works may be otherwise injurious. 



While the accumulative action is going on, every abrupt projection 

 from the coast i^ an im|)e(linient to the progressive motion of the 

 beach until its angle is tilled np. Such abrupt projections offer no 

 protection against the destructive ac'ion; when, therefore, by the in- 

 crease of wind, the action of the st-a becomes violent, an accumulation 

 previously caused by a projecting jier is rapidly removed, and again 

 is rapidiv deposited where it is not resisted. And there is perhaps 

 no combination of circumstances less capable of resisting, or more 

 favourable to the deposition of, the shingle, than is found in artificial 

 harbours, shielded by an ahrupl weather pier in a line of beach. 



With a long continuance of violent winds from the same quarter, 

 every accumulation of loose shingle is broken down, and is hurried 

 forward, while it unremittingly appears to seek protection. During 

 the recent gales every inlet within the tract of the beach was seriously 

 encumbered with it ; commenced with the heap accumulated by the 

 very pier that was inteiided to prevent such an effect (where such ex- 

 istedj, and increased by the successive arrivals of those more remote, 

 together with that quantity commonly passing along the sloping plane, 

 but BOW brought down by the. destructive action and forced along with 

 accelerated motion. « » 



Manv very interesting facts might be mentioned concerning the 

 effects produced by the continued gales at various places on the coast, 

 but I find that the description of them in sufficient detail to make them 

 useful would extend this paper much beyond the limits assigned : I, 

 however, trust that the reference to two of the most remarkable cases 

 will be found sufficient to illustrate the principles attempted to be ex- 

 plained. 



Section 7. 



The only natural power by which the channels through the beach 

 are retained, is the returning force of the water, which on this coast 

 is generally scanty. And it is obvious, that however judiciously that 

 force may be employed, it is but remidial in principle, and necessarily 

 implies a previous evil. So long, therefore, as the cause continues to 

 act, the remedy is prevented, and the harbour becomes inaccessible 

 when protection is most required. 



If on inspection of the great bank recently thrown up at Dover, 

 we imagine it to be dispersed over several miles of the sloping 

 plane, and assume the whole to be in continued and equable motion, 

 it will immediately be inferred, that the quantity that would be 

 passing a civen spot at one time would be comparatively insignifi- 

 eant; and hence, since we have no reason to suppose that there will 

 be a limit to the quantity, and since it has been shown that its 

 motion cannot be prevented, it follows that the great objects in view 

 must be attained, first, by securing permanently such accumulations as 

 are necessary for the protection of land from the action of the sea, or 

 xiseful by their addition to its surface; and secondly, by facilitating 

 and inciting the progressive motion of that superfluous quantity from 

 whence the evils complained of are derived: and therefore the unin- 

 terrupted and permanent welfare of the numerous harbours which 

 communicate with the sea, through the extensive tract of the shingle 

 beach, is dependent more on a system 0/ management along l/ie coast, 

 than upon particular devices adapted exclusively to each separate 



Engraving upon Metals. — M. Mellon! lias announced to tlic Kiencli Academy 

 llint M. Cirelli, of Naples, has been able to obtain plates upon metals by gal- 

 vanoplaslic methods. His discovery is to form imnieiliately the plale com- 

 pletely engraved after a simple design. M. Melloni lias submitted some of 

 the plates to ' be inspeciion of the Academy. Tlic process is not detailed, as 

 Cirelli !S preparing to secure a patent for it. 



PREVENTION OF EXPLOSION IN .STEAM ENGINE 

 BOILERS. 



The Gold lait Medal was presented by the Society of jdrli to Mr. Robert 

 M'Enen, Glasgon;,/or his Double Mercurial Safety-P'alre for Steam 

 Engine Boilers. 



There are two evils against which it is especially necessarv to pro- 

 vide in the construction of an apparatus for preventing explosion in 

 boilers, viz. the possibility of the steam passage being intentionally 

 closed, for the purpose of obtaining extraordinary pressure ; and the 

 failure of the self-action of the apparatus through the accidental de- 

 rangement of its parts. 



Mr. M'Ewen's apparatus consists of a pair of open tubes, the ends 

 of which are immersed in mercury contained in cups connected with 

 the boiler by a pipe. At the junction of this pipe with its branches 

 for the two cups, is a three-way cock, the ports of which are so pro- 

 portioned to the openings of the branch liipes, that the steam can 

 neither be opened on, nor cut off from, both cups at the same time. 

 The mercury tubes are proportioned in length to the greatest pressure 

 which the boiler will bear with safety ; the mercury will therefore be 

 blown out of the acting tube into the dome at the top, whenever the 

 pressure exceeds this limit, and will fall down through the other tube 

 into the empty cup, while the steam blows out through a pipe at the 

 top of the dome.* When the pressure is sufficiently reduced, the cock 

 may be turned, and the cup which was last filled becomes the acting 

 side of the apparatus. 



On the 'th of April, a committee of the Society inspected the action 

 of Mr. M'Ewen's mercurial valve, the apparatus having been attached 

 to the boiler at the works of Messrs. Fairbairn and Murray of Mill 

 Wall. The steam was opened on the mercury at a pressure of five 

 pounds to the square inch, and as soon as it attained the pressure cor- 

 responding to the length of the tubes, viz. seven pounds, the mercury 

 was blown, without any loss, into the dome and fell into the empty 

 cup, while the steam blew out through the pipe at the top of the 

 dome, and was condensed in a vessel placed to receive it for the pur- 

 pose of experiment. On examination of the water in this vessel, not 

 a particle of mercury was found in it. This result sufficiently proved 

 the efficiency of the pipe, which is produced to some distance down- 

 wards within the dome, as represented in the section fig. 1, for the 

 purpose of preventing the mercury from splashing out with the rush 

 of steam. 



As the action of this apparatus depends simply on a. physical princi- 

 ple, viz. the opposition of the elastic force of steam to the static pres- 

 sure of mercury, w ithout the intervention of a mechanical obstruction 

 of any kind, it cannot fail of acting, so soon as the pressure of steam 

 exceeds the limit corresponding to the length of the tubes. The no- 

 velty of the invention is in the employment of a mercurial tube as a 

 safe vent for the steam, these tubes having hitherto been used only as 

 indicators of steam pressure, being long enough to allow the steam to 

 attain a dangerous pressure without relieving it or giving any other 

 notice of the fact than what may be observed by the eye. 



REF£RE^•CE TO THE FIGUUES. 



Figure 1 represents the whole apparatus in section. .\ the pipe 

 connected with the steam boiler, B the hollow plug of a cock with a 

 side opening at c, through which the steam passes into the area D, 

 and pressing on the mercury causes it to rise in the tube E till its 

 weight counterbalances the force of the steam; the tube E opens into 

 the chamber and dome F, to which there is free access for the atmo- 

 sphere through the neck u ; if, therefore, the steam should at any time 

 exceed the due pressure which is limited by the length of the tube E, 

 it will drive all the mercury before it up this tube into the chamber F, 

 and will escape thiough the neck t, ; in the meantime the mercury will 

 enter the opposite tube H through the small hole i, and flow down into 

 the other vessel J, where it will be ready again to act as a safety-valve 

 as soon as the attendant has turned round the plug B by its handle K, 

 thus cutting off the communication of the steam with the vessel d, and 

 opening it into the vessel j. The construction of both sides of the 

 apparatus being exactly alike, the tube E having an aperture at l to 

 receive the mercury from the chamber F, this operation may be re- 

 peated .as often as the escape of the steam gives notice of its being 

 necessary. The bottom of the chamber F, though straight from L to I, 

 is curved like a trough in the cross diameter, as shown by the curve 

 under f, to conduct all the mercury through the hole i or l, whichever 

 may be opposite the acting tube. 



• Mr. Il-Ewen intends that an alarm-whistle le placed in this opening, 

 and also that the apparatus serve as a gauge for indicating the variation of 

 pressure, by means of graduated float-rods in the mercury lul»s. 



