Jtjne 1, 1895.] 



KNOWLEDGE 



137 



when the solar rays pass clear through cirrus. " Sun- 

 dogs " appear to be formed when the refracting crystals 

 are limited to a small area of cloud. Parhelia, in hot 

 weather, usually develop a patchy mass of prismatic light, 

 which sailors would call " sun-dog." The subject is a 

 wide one. — S. Baruee. 



MIEAGE I^^ THE DESERT. 

 To ilie Editor of Knowledge. 



SiE, — I wish to know in what position the sun is 

 during the phenomenon of a mirage in the desert '? Is 

 it opposite to the appearance, as in a rainbow '? Is the 

 sky of the desert land blue or a glare of white '? (I am 

 not alluding to sunrise or sunset.) Is colour traced in 

 the mirage, or merely a neutral, misty appearance ? I 

 have heard statements made contradictory one to the 

 other. People seem to know very little on the subject. 



Yours truly, 



(Miss) F. Stephens. 

 Fern Cottage, Lincoln Eoad, Dorking. 



[In the mirage of the Sahara and other arid deserts, the 

 soil has become heated by the presence of the sun, and the 

 prospect seems bounded by a general inundation, not 

 depending upon the exact position of the sun as in the 

 rainbow and mountain - giants. The observer sees in 

 the distance the reflection of the sky and of terrestrial 

 objects, as on the surface of a calm lake. The terrestrial 

 objects have, of com-se, an inverted image. The distance 

 of the apparent water is so great that the colour of the 

 sky and terrestrial objects cannot be discriminated, any 

 more than at a distance the inverted images of trees are 

 seen in a Highland lake. The villages beyond appear as 

 islands in the midst of a great lake. Under each village 

 its inverted image is seen as it would be if reflected from 

 the surface of a sheet of water. On approaching, however, 

 the deceptive inundation recedes, and the reflected image 

 vanishes, to be succeeded by another, as some more 

 distant object comes in sight. It is explained by the 

 heating and consequent rarefaction of the air in contact 

 with the hot soil. The density in the lowest stratum 

 of air (a foot or so in thickness) increases upwards, and 

 rays entering this stratum at a small inclination are 

 bent upwards in a manner resembling " total reflection," 

 but with the corner rounded off. Skylight rays descending 

 become bent upwards ; the eye receives an impression 

 resembling that produced by the reflection of skylight 

 from water. The illusion is rendered more perfect by the 

 flickering due to convection currents, which causes an 

 appearance like a breeze playing over the illusory water. — 

 J. G. McP.J 



Notices of ^Soofts. 



Molluscs and Braclwqiods. By the Eev. A. H. Cooke, 

 M.A., A. E. Shipley, M.A., and F. E. C. Eeed, MA. Pp. 

 635. (Macmfllan and Co.) 17s. The Cambridge Natural 

 History, of which this volume is the first published instal- 

 ment (though it is really the third volume in a series of 

 tenj, is intended, to quote the publishers' circular, " in the 

 first instance, for those who have not had any special 

 scientific training, and who are not necessarily acquainted 

 with scientific language. At the same time, an attempt is 

 made, not only to combine popular treatment with the 

 latest results of modern scientific research, but to make 

 the volumes useful to those who may be regarded as serious 

 students in the various subjects. Certain parts have the 

 character of a work of reference." We think, however, 

 judging from the volume before us, that the general reader 



will find the series unsuitable for his mental digestion. 

 For instance, on the third page, a tropical beach is 

 described in this manner : "On the rocks at high water 

 mark, and even above them, occur TruncatcUa, Mclampm, 

 Littorina, and Siphoiiariu ; where a mangrove-swamp 

 replaces the rock, on the branches overhead are huge 

 Littorina, while three species of Cei-ithidia crawl on the 

 mud, and Cijrena and Area burrow into it. Lower down, 

 in the rock pools at half tide mark are Ccrithium, Purpura, 

 Oiiijilialius, Anacliis (2 sp.) JVassa, and several Crepidula." 

 The description runs on in the same strain for a whole 

 page. Of course, it cannot be held for a moment that 

 readers unacquainted with malacological nomenclature can 

 have the faintest interest in such an enumeration of forms 

 of moUuscan life, and by no stretch of imagination can it 

 be regarded as a pabulum in which the public " who are 

 not necessarily acquainted with scientific language " will 

 find delight. But though the work does not appeal to those 

 who like to assimilate knowledge almost unconsciously, it 

 is undoubtedly readable in many of its parts, and is far 

 better illustrated than any work covering the same 

 ground. 



Mr. Cooke is responsible for the main part of the 

 present volume, but the treatment of recent and fossil 

 Brachiopods has been entrusted to Mr. Shipley and Mr. 

 Eeed respectively, both of whom are leading specialists 

 in those groups. After an introductory statement of the 

 position of moUusca in the animal kingdom, and the 

 basis of classification into Cephalopoda, Gasteropoda, 

 Schaphopoda, and Pelecypoda, Mr. Cooke describes the 

 convenient grouping into Glossophora and Aglossa. The 

 subsequent arrangement of matter is noteworthy and 

 commendable. Not until the habits and general economy 

 of mollusca have been fully considered are the animals 

 systematically treated, and their geographical distribution 

 considered. A point worth mention relates to the use 

 of snails as an article of food. Judging from Mr. Cooke's 

 account, it would seem that in this country snails are 

 very rarely eaten. We have, however, seen dishes piled 

 high with snails in the oyster bars of Bristol, and have 

 eaten and enjoyed them there, and in other parts of the 

 West of England. Mr. Shipley's chapter on recent 

 Brachiopods is chiefly concerned with the anatomical 

 structure of the animals. Indeed, little can be said about 

 the habits and natural history of the Braohiopoda. The 

 group owes its chief interest to the immense variety of its 

 fossil forms throughout the whole series of geological 

 formation, and Mr. Eeed briefly reviews the chief 

 characteristics of those genera which have the greatest 

 geological importance. Having sketched out the lines upon 

 which the volume has been constructed, it only remains 

 to be said that if succeeding volumes are like this one, the 

 Cambridge Natural History will rank as one of the finest 

 works on natural history ever published. 



Froi/ress of Scicnci'. By .1. Villin Marmery. Pp. 358. 

 (Chapman and Hall.) 7s. 6d. Whoever is responsible for 

 the series of pufts pasted inside the cover of the copy 

 of this book received by us fur review, whether it is the 

 author or the publishers, should know it is in very bad 

 taste. Surely it is for the reviewer to decide whether the 

 book is "remarkable for accuracy," or exhibits "great 

 exactness " ; and it should be left for him to say that " the 

 work will prove the most instructive of its kind." The 

 least that can be said about such action is that it is calcu- 

 lated to do the book more harm than good. The place for 

 the fourteen numbered paragraphs setting forth the claims 

 of the book to favour is not inside a review copy. 



Putting this matter aside, we are of opinion that the 

 book will be found handy for reference, though it is by no 



