362 



NATURE 



[February i8, 1892 



pendulum experiments pure and simple, we have memoirs 

 by Da la Condamine, Borda and Cassini, Prony, Kater, 

 Bessel, Sabine, and Baily, and in addition to these we 

 have Stokes's paper on the effects of fluid friction on 

 the motion of pendulums. 



We must, in conclusion, express our gratitude to La 

 Society Frangaise de Physique for the publication of 

 these volumes, and to MM. Potier, Joubert, and Wolf, 

 for the masterly way in which they have edited them. 

 We hope that the volumes before us are but the first 

 terms in an infinite series. J. J. Thomson. 



THE FORMATION OF BEACHES. 

 Sul Regime delle Spiaggie, e sulla Regolazione del Porti. 

 (" On the Formation of Beaches, and the Rules for de- 

 signing Harbour Works.'') By Signor P. Cornaglia, late 

 Inspector of the Royal Corps of Italian Civil Engineers. 

 (Turin : R^ Tipografia Paravia, 1 891.) 



THE laws that govern the movements of sand along the 

 sea-shores, the formation and corrosion of beaches, 

 the shifting of bars and sand-banks at the mouth of rivers, 

 and the silting up of harbours, have not yet been clearly 

 explained, as the problem is a complex one. These 

 effects result from a combination of various causes, such 

 as the action of sea- waves, of tidal currents, of the natural 

 discharge of rivers, and of the dimensions and specific 

 gravity of the materials that form the beaches and give 

 ' them a special angle of rest. 



To examine this problem, Signor Cornaglia takes it up 

 in its most simple form. He tries to ascertain the laws that 

 regulate the propagation and effects of sea-waves acting 

 in a tideless sea, or nearly so, as is the Mediterranean. 

 These researches are made both theoretically and practic- 

 ally, by the help of mathematical analysis and by verify- 

 ing the results of theory by direct observations of waves 

 in accumulating the materials that form the beaches. 

 These form the subject of two memoirs, in which are 

 discussed the vertical propagation of waves in liquids, 

 and the origin and action of bottom- waves {flutti di fondd) 

 in liquids in a state of undulation. 



The results of these researches are thus expressed : — 

 id) The undulatory movement of liquids generates near 

 the bottom an oscillatory movement called bottom-wave, 

 or under wave {fliitto di fondo), which is alternately 

 directed to or from the shore ; (b) vertically under the 

 crest of a superficial wave, the bottom-wave is directed 

 toward the shore ; under the hollow of a wave it is 

 directed toward the sea ; {c) the force of these bottom- 

 waves increases with the force of the superficial waves, 

 with the greater distance or ''^ fetch " from which the waves 

 arrive, and with the greater depth of the sea ; id) the 

 energy of these bottom-waves may be great at great 

 depths ; {e) on a rising submarine slope the force of the 

 bottom-waves directed toward the shore is greater than 

 that of the reverse bottom-waves ; (/) the bodies resting 

 upon the bottom of the sea and exposed to these bottom- 

 waves are struck in alternate directions ; {g) the com- 

 ponent of the weight of these bodies, parallel to the bottom 

 slope, may counterbalance the effects of the direct 

 bottom-waves, or, added to the effect of the return 

 bottom-waves, these latter may prevail upon the direct 

 NO. I 164, VOL. 45] 



ones ; {h) the line along which the action of each of the 

 two opposite bottom-waves, combined with the component 

 of the weight of the bodies, counterbalances that of the 

 other, is called the neutral line j [i) all conditions being 

 equal, this neutral line is situated at a greater depth the 

 stronger the waves are, and the smaller the slope of the 

 bottom and the size and specific weight of the bodies 

 resting upon the bottom ; (/) in the Mediterranean this 

 neutral line is situated at depths varying from 8 to 10 

 metres, or 27 to 33 feet ; {k) on the land side of the 

 neutral line the materials forming the bottom of the sea 

 are pushed by the bottom-waves toward the shore, on 

 the sea side they are drawn toward the greater depths ; 

 (/) parallel to the shore the materials travel always in a 

 contrary direction to that from which the waves come, 

 and they travel alternately in one way or the other 

 according to the direction of the waves. However, the 

 ultimate direction in which these materials move is that 

 corresponding to the prevailing waves, which is also, more 

 or less, that of the prevailing winds. 



In a third memoir the author discusses the formation 

 or corrosion of beaches in a tideless sea, and explains 

 a long series of observations and experiments made prin- 

 cipally by himself along the shores of the Riviera, with 

 a view of ascertaining the position of the neutral line. 



In a fourth memoir are examined the conditions of the 

 estuary of a river opening into a tideless sea, and the effects 

 due solely to the waves having been previously ascertained, 

 the effects due to the outgoing water of the river are de- 

 scribed. Then the author passes on to examine the case of 

 a river opening into a sea subject to the influence of 

 tide and tidal currents ; and by separating the effects of 

 the waves and of the outgoing water of the river proper, 

 he tries to explain the effects due exclusively to the action 

 of tides. From this he gives some hints about the 

 probable position of the neutral line resulting from all 

 these different causes, and by comparing the conditions 

 of the Mersey with those of the Thames and other tidal 

 rivers, the author concludes that in the former case the 

 outlet of the river is on the land side of the neutral line, 

 and thus the materials brought down by the river or 

 washed away from the coast are pushed into the estuary, 

 while in the case of the Thames its outlet is on the sea 

 side of the corresponding neutral line, and the materials 

 are drawn into deeper water. In the same way he explains 

 why some harbours in the Mediterranean having their 

 entrance in depths of water superior to 10 metres, and 

 thus outside the neutral line, have kept good for centuries, 

 while other harbours with their entrance inside the 

 neutral line have gradually silted up. 



Having thus explained his views on the formation of 

 beaches, Signor Cornaglia makes use of them in laying 

 down in several memoirs the principal rules for designing 

 such maritine works as may be necessary to maintain or 

 improve the navigable channel in the estuary of a river, 

 or to protect the entrance of a harbour, or to prevent the 

 sea from encroaching upon a beach. There are descrip- 

 tions of works carried out in several Italian harbours 

 during this century, and the results, good or bad, accord- 

 ing to the way these works were designed. 



A drawback to this book is that it is written in the form 

 of separate memoirs, so that there are many repetitions 

 and some unnecessary details, which rather diminish than 



