174 



DISCOVERY 



One particular application of aerial photography, 

 which will be of tremendous use to all pilots, is the 

 provision of aerial photographs of all aerodromes on 

 the international airways. It frequently happens that 

 a plan, even on a comparatively large scale, does not 

 give the pilot the information he desires, and particu- 

 larly when landing by night on a strange aerodrome 

 he is in great difficulties owing to his lack of knowledge 

 of the landing area itself and the surrounding buildings 

 and obstacles. An aerial map, possibly composed of 

 only one photograph, will show him all details he 

 requires in an instant, and he will experience little 

 difficulty, even in darkness, in selecting a suitable 

 spot at which to glide into the landing ground. The 

 Air Ministry now issues periodically very detailed 

 particulars of all aerodromes known to be in use 

 throughout Europe, and frequently issues plans with 

 the descriptions, but in no single case have aerial 

 photographs been circulated, though in many 

 cases these are available for inspection at the Air 

 Ministry. The " Pilot Book " of civil airmen of 

 the future will undoubtedly contain these photo- 

 graphs, together with other particulars which may 

 be necessary. 



During the war one somewhat peculiar use was 

 found for aerial photography, which use may be of 

 assistance to civil pilots when flying in bad weather. 

 It was noticed that different areas had very distinctive 

 types of country when seen from the air. Thus, 

 certain parts of Flanders had the fields arranged, in 

 the majority of cases, in the form of very regular 

 rectangles. Farther south the fields altered somewhat 

 in shape. In other parts, as in England, they were 

 irregular and generally hedged or bounded by trees. 

 Still other parts of the country were thickly wooded ; 

 others were mountainous. In all cases this distinctive 

 character was found to e.xtend over certain definite 

 areas. Photographs of typical country were therefore 

 taken and supplied to imits, together with sections 

 of the map showing the areas over which each type of 

 country extended ; notes were also supplied giving 

 the " colour schemes " of the land. Pilots were then 

 taught to memorise the geographical positions of the 

 various types of country, so that if they were lost 

 through flying for long periods above the clouds or in 

 misty weather, they could gain some idea of their 

 position when they saw the ground again by noting 

 its characteristics. This idea might well be adopted 

 for civil pilots, though under present conditions, when 

 flying on the airways oniy takes place in fairly good 

 weather, there is little possibility of a pilot losing 

 himself. In the future this method may become of 

 more use, particularly when many private owners use 

 the airways, in addition to the regular pilots who fly 

 only along certain routes. 



The Biology of Coral 

 Reefs 



By F. A. Potts, M.A. 



Lrcliir^T in ZooUkjij in the University 0/ Cambridge 



The coral reefs of the tropics are the most character- 

 istic assemblages of sessile animals in the world. They 

 grow in such shallow and clear water that many a 

 traveller is able to bring back an enduring impression 

 of the beauty of these water gardens from some far- 

 away harbour. And for a trained biologist the field 

 offered for observation and experiment is without 

 equal. 



A Pacific coral reef, like that represented in the 

 accompanying photograph (Fig. i), gives the same idea 

 of luxuriant life as a tropical forest. It is hard to 

 realise that the corals which compose it are animals, 

 and that they subsist entirely on animal food. Their 

 peculiar physiological habit of excessive lime secretion, 

 combined with their powers of vegetative reproduction, 

 has enabled them to build, in the course of countless 

 generations, these vast platforms throughout the 

 tropic seas. In each colony there exists a living film, 

 over the massive skeleton of carbonate of lime, com- 

 posed of innumerable individuals, resembling, in the 

 structure of their soft parts, tiny sea-anemones. 



The variations in form of the skeletons of coral 

 colonies are exceedingly striking and beautiful, and 

 parallel every extravagant form of which plant life 

 is capable. Not only do genera and species differ 

 markedly, but within species the variability' is so great 

 that their determination is a matter of the greatest 

 difficulty. How far the form of coral colonies is a 

 reflection of the action of external forces such as the 

 wash of the sea and the light of the sun is a problem 

 which has engaged the attention of several biologists 

 of late years. Particularly, since the foundation in 

 1904 of the Department of Marine Biology of the 

 Carnegie Institution, intended to concern itself with 

 the investigation of tropical seas, this and other 

 problems of coral reefs have been attacked with vigour. 

 The fastidious nature of reef corals is a common- 

 place of zoological and geological textbooks. For 

 vigorous growth the water must not be too deep, 

 and they will not grow at all below 25 fathoms. The 

 temperature must be uniformly high ; the average 

 for the coldest month of the year must not be below 

 22° C. They can only grow in the clearest ocean water, 

 containing abundant supplies of small animal " plank- 

 ton." If the water is at times considerably agitated, 

 then the strongest and most rapid growth occurs. 



Within these limits there is, however, a good deal 

 of variation, and on a coral reef widely differing 



