May 13. 1909J NATURE 



323 



Besides the existing Passes, it sliould be noted that 

 there seems a hkehhood of at least throe more bemg formed 

 —at Camp Frigate the mangrove swamp extends right 

 through the island to the sea, and no doubt a certam 

 amount of water already traverses the land at that pomt ; 

 in Polymnie Island, at one place the swamp is withm 100 

 yards of the shore, and a Pass will probably be formed in 

 time; at Uune Jean Louis there is only a quarter of a 

 mile between the sea and the swamp, and if the lagoon 

 erosion continues no doubt -Main Island will be divided 

 at this point. It is worthy of note that fresh Passes seem 

 always to be formed by lagoon erosion, and not from the 

 seaward side. 



The lagoon itself is very shallow, and the bottom sandy 

 in the iiuddle, changing into fine mud as one approaches 

 the mangrove swamp. Everywhere one is forcibly struck 

 by the extent of the erosion in the lagoon. Judging by 

 it's maze of small islands and mushroom-shaped rocks, at 

 least one-third, or even more, of the lagoon can be shown 

 to have been land at one time. At spring tides the amount 

 of fine mud carried out to sea in suspension is very 

 large, and it is obvious that the lagoon is still growing 

 in "size. There is some difficulty in accounting for the 

 rapid transformation of the rock into mud, as boring 

 animals are not common ; I would suggest that possibly 

 the mangroves have some further action on the rock than 

 merely helping to split off large pieces. 



As regards the vegetation, it is impossible to say much 

 until the specimens collected have been worked out. The 

 mangrove swamps extend right round the lagoon sides 

 of the islands, Rhizophora and Brugiera being the pre- 

 dominating genera, though there is also a large quantity 

 01 Ceriops. Rhizophora seems to require a deep mud, but 

 Brugiera thrives better in the more rocky places and on 

 the small islands. In the extreme east of the atoll there 

 is a large forest of the pseudo-mangrove Avicennia. The 

 onlv other fact that need be mentioned is that Esprit 

 Island has several plants not found elsewhere, or which 

 are common to it and Picard Island alone. 



The fauna also must be left until the collections arrive 

 and have been examined. So far as can be seen at pre- 

 sent, it appears to be of the regular coral-island type, 

 with such additions in the land animals as would be 

 natural considering the large amount of land and the larger 

 flora. It should, however, be remarked that the man- 

 grove swamps were very disappointing in their fauna, a 

 condition very different from that described in mangrove 

 swamps in other localities. 



Large numbers of giant land-tortoises still exist, but 

 the problem of their distribution does not relate to 

 Aldabra alone, as I have found their remains on Assump- 

 tion and Cosmoledo, and they are also known to have 

 occurred in nearly all the Seychelles islands, two of which. 

 Bird and Dennis, are coralline in structure. 



In conclusion, I would suggest that the reefs and 

 islands of the .Aldabra-Farquhar line present a most 

 interesting series in the possible life of an atoll. 



fa) Asiovc. — Land rim of atoll almost perfect, and 

 mostly rocky. Only one small Pass of recent date. 

 Lagoon exceedingly shallow, but getting rapidly deeper. 

 Formation of another Pass proceeding. 



(!)) .IWafera.— Land rim still very perfect, and mostly 

 rocky. Several Passes already in existence. Strong 

 evidence of increase of lagoon at expense of land. Lagoon 

 deeper, and at least three Passes in course of formation. 



[c\ Cnsmoledo. — Land rim broken up into a series of 

 small islands only. Most of encircling reef bare, but 

 evidence of a former rock-cap in mushroom-shaped rocks 

 and minute islands. .\ noticeable increase of sand on 

 the island, and decrease of rock. Lagoon deeper than 

 that of .Mdabra, and more open. • 



(d) Farauhar. — Judgin;; from your description,' land rim 

 very small. Island nearly all sand, and typical coral rock 

 very scarce. Lagoon still more open. 



(c) A final or hypothetical stage may be imagined as 

 an nfnil with a considerable lagoon, without, perhans. any 

 land, or, if land is present, only as sand cays piled up 

 on the reef. 



J. C. F. Frvkr. 



1 Stanley Gardiner. T an=. T.inn. Soc, xi!., pp. i40-5- 



NO. 2063, VOL. 80] 



ROW MOTORS AND PROBLEMS CONNECTED 

 WITH THEM. 



rHE " James Forrest " lecture vvas delivered at an extra 

 meeting of the Institution of Civil Engineers on 

 April 26 by Colonel H. C. L. Holden, R.A., F.R.S. The 

 author took for his subject the road motors of the pre- 

 sent day, and so.ne unsolved problems connected with 

 them. The modern era of mechanically propelled road 

 vehicles was inaugurated with the passing of the Act. 

 of 189b. In the case of steam traction engines there has 

 since then been no rapid progress ; problems awaiting 

 solution in this class are ia) greater economy of fuel; 

 (6) means of condensing all or part of the water converted 

 into steam ; (c) reduction of weight and increase of 

 adhesion surface to the road with minimum pressure per 

 unit area on the road, without sacrifice of other qualities, 

 including speed. 



The problems regarding medium and light road motors 

 are similar, and the author devoted most of the paper to 

 the latter type, which includes passenger vehicles, light 

 delivery vans, and motor-bicycles, tricycles, &c., the speed 

 of which is limited by law to twenty miles per hour. 

 Though by far the greater number of such vehicles are 

 driven by petrol engines, there are others driven by steam, 

 electricity and combined systems ; compressed air and 

 liquid air have also been tried, but have never passed 

 the experimental stage. It is estimated th.at, of the 

 100,000 motor-cars and cycles in use in the British Isles, 

 more than 99 per cent, are driven by petrol engines. In 

 spite of the great inherent advantages of the steam engine, 

 steam-driven road motors form so small a proportion of 

 the whole as to render it obvious that at present their 

 advantages do not outweigh their disadvantages. For 

 heavier goods and public passenger vehicles they may in 

 time compete with petrol vehicles, but for private motor- 

 cars the boiler and burner will always, in the authors 

 opinion, handicap the system's other advantages. Electric 

 propulsion would be ideal if a source of electricity were 

 available which would bear comparison with petrol in 

 weit'ht, cost, and portability. The generation of electricity 

 direct from the oxidation of coal or other fuel cannot be 

 said to be impossible, and it may be actually within our 

 reach if w-e only knew how and where to grasp it. 



Nearly all the internal-combustion engines in use employ 

 the Beiu-de-Rochas or four-stroke cycle. Greater uni- 

 formity in the turning moment has been secured by having 

 multiple cylinders, and at high speeds of rotation there 

 would not appear to be much room for improvement in 

 this respect. At the same time, individual impulses are 

 not entirely damped out before they reach the road, and 

 these leave their effect to a certain extent evident on the 

 wearing surfaces of the tyres. It is difficult to see how 

 a more uniform turning moment can be obtained with 

 reciprocating engines. An engine of the rotary or turbine 

 type would be effective, but no successful example at pre- 

 sent exists. The moving parts of the engine, and also 

 the explosion pressures on the pistons, can be balanced 

 so perfectly by use of the double-piston type that, if the 

 car is at rest, it is diflicult to detect by eye or ear if the 

 engine is running. The turning moment, however, cannot 

 be^balanced, and reacts on the whole car when running, 

 especially at full power and slow speed, as when climbing 

 a hill A more perfect solution may be obtained at some 

 future time but it would necessitate the employment of an 

 entirely different type of engine and transmission 

 mechanism. 



The use of the spray or jet carburettor has now become 

 universal. The carburettor that will supply a constant 

 mixture at constant pressure and temperature under all 

 conditions of running of the engine is one problem which 

 has vet to be solved. When to this has been added some 

 device whereby this constant mixture shall be diluted with 

 air to exactly 'the correct extent to give perfect combustion 

 on explosion in the cylinder, then, and then only, will 

 perfection in this matter have been reached. The whole 

 question of carburation is very complex, and the author 

 regards with satisfaction the fact that it has now been 

 taken up seriously by scientific experimenters. _ 



No universal Method of ignition has been arrived at, 

 though electricity in some form or other seems more nearly, 



