December 1, 1892.] 



KNOWLEDGE 



231 



differing for different materials, to account for tlie different 

 velocities of repulsion, and the different curvatures of 

 cometary tails. 



The particles of which comets' tails are composed are 

 endently very small, for they are small in average diameter 

 compared with the wave-length of light — tliis is rendered 

 endeut by the fact that the ligh,. dispersed of comets' tails 

 (except in the neighbourhood of the nucleus, where a 

 gaseous spectrum is recognizable) is generally strongly 

 polarized — and it seems to me that we may satisfactorily 

 account for the repulsion of such small particles from the 

 snn without calling to our aid any unknown force, but by 

 merely considering the repulsion which would be caused by 

 evaporation taking place from the side of the particles 

 exposed towards the sun. In a paper published in 

 Knowledge for February 16th, 1883, I have discussed 

 the accumulated effect of the minute recoils which must 

 accompan}' evaporation as one molecule after another is 

 thrown off' from the surface of a small heated body towards 

 the sun. When three-fourths of a heated particle have 

 been thus evaporated towards the sun, the velocity of 

 the remaining one-fourth away from the sun would be 

 greater than the molecular velocity of evaporation, but the 

 irregularities in the tails of comets referred to above show 

 that we have other forces to consider, which slightly vary 

 the form that cometary tails would assume if the matter of 

 the tail were only acted upon by repulsive forces, in 

 directions awav fi'om the sun and awav from the nucleus. 



Scicnct Xotrs. 



Part XIII. of •' The Old and New Astronomy," contain- 

 ing the last portion of the chapter on the SteUav Universe, 

 with the title page, preface, and index, is now in the hands 

 of the public. .^, 



Fresh applications of photography are continually being 

 reported. The camera has revealed much with regard to 

 animal locomotion ; the nature of the horse's move- 

 ments in trotting and galloping have been revealed, much 

 to the surprise of many animal .artists. So with regard to 

 the movements of birds' wings, which have been photo- 

 graphically registered. But photography lias recently been 

 applied to record the movements of the growing parts of 

 plants, and some curious results have been obtained. In 

 the case of the hop-convolvulus, the movement of the 

 young stems consists of a succession of irregular, circular, 

 or elUptical curves, which vary every moment, even in 

 their direction. These movements are caused by unequal 

 growth in dift'erent parts of the stem. Even when asleep, 

 plants move, not, as was formerly supposed, interruptedly, 

 but continuously. . ., 



The apple-rot is produced by a parasitic fungus, which 

 appears to be the same as that which causes the ripe-rot 

 in grapes. The disease is infectious, as anyone who keeps 

 a number of apples may observe ; for it is passed on from 

 one apple to another. But if those who gather the apples, 

 and pack them for market, were a little more careful in 

 handling them, we should not have so many affected. 

 Any slight bruise on the skin is sufficient to cause those 

 disagreeable brown spots which cannot be eaten. 



In a recent number of Xciturc, Mr. H. H. Dixon de- 

 scribes the walking of certain insects {Artkmpoda) as 

 revealed by photography. The paper is most interesting. 



Congress has this year refused, for the present, to 

 sanction any further expenditure on the United States 



Geological Survey. Field work has been suspended, but 

 it is to be hoped that the survey will not come to an end, 

 for such an event would be little short of a catastrophe. 

 Enghsh geologists have learned a great deal from the 

 splendid monographs and bulletins that have been so 

 generously supplied to our libraries and museums, and all 

 must admire the ability and thoroughness with which most 

 of the work has been done. America is such a huge 

 country, and contains such magnificent illustrations of 

 physical features produced on a great scale by denudation, 

 volcanic action, earth-movements, &c., that it would be a 

 great pity if no further geological surveying were under- 

 taken. It is well known, however, that certain abuses 

 have been going on, and, if these can be removed, probably 

 this important work will some day be renewed. 



•-•-« 



An interesting discovery is reported in the November 

 number of the Jfalitiiranean Xatiiralixt by Mr. J. H. 

 Cooke, namely, the finding of a portion of an Arctic bear's 

 jaw I Ursu.1 a'rcto:<} in pleistocene strata, Malta. It had 

 prenously been concluded, from the fact that elephants' 

 bones discovered there were much gnawed, that there must 

 have been carnivorous animals living there at that period, 

 but no remams of such could be found. Now Mr. Cooke 

 has earned out some excavations in the Har Dalam 

 cavern, and found the evideucfi^that was wanted, viz., an 

 entire ramua of the lower jaw of a bear with its canine 

 molars. Besides the bones of elephants, those of the 



hippopotamus, stag and a large dog were foimd. 

 — •-♦-, — 

 A new genus and species of blind cave salamander is 

 reported by Mr. L. Stejnger in the " Proceedings of the 

 United States National Museum." The discovery of such 

 an animal in North America is regarded by the writer as 

 a most important and interesting event. 



« ♦ ■ 



Scie/ice contains an mteresting paper on the food of 

 humming-bu-ds by Dr. Morris Gibbs. Although he has 

 dissected many specimens, he found no evidence that they 

 lived on insects. Possibly, when flowers are scarce some 

 species of insects may be taken, but when flowers are 

 abundant the ruby-throat of Michigan lives on honey. 



The important work by Mr. SaviUe Kent, on " The Great 

 Barrier Eeef of AustraUa," announced by Messrs. 'W. H. 

 Allen & Co., is a book to which naturalists are looking 

 forward with considerable interest. Geologists will also 

 be interested in it, both on account of the problems 

 connected with Darwin's theory of coral-reefs (now 

 I abandoned bv ilr. ilurray and others), and because there 

 1 are fossil reefs— probably of the same nature— buried in 

 several well-known English formations, viz., the Silurian, 

 devonian and carboniferous, and the Jurassic series. But 

 for some reason or other the biological aspects of coral- 

 reefs have been somewhat neglected, although their pools 

 and lagoons are full of varied forms of life. The author, 

 who is fishery inspector of several of the Australian 

 Colonies, is a recognized authority on lower forms of life, 

 such as infusoria, sponges, and corals. The book will be 

 illustrated by 16 coloured quarto plates and 48 photo- 

 graphs. The" former have been copied from paintings from 

 nature by the author. The photographs show large areas 

 exposed at low tide, and some submerged parts of reefs. 



A very wonderful new " extinct monster " has recently 

 been set up in the Natural History Museima, CromweU 

 Road. It is to be seen in a handsome glass case at the 

 end of the fossil reptile gallery, among the ichthyosauri 

 and other extinct types. This fine specimen, the only 

 complete skeleton known, was discovered last year by 

 Professor H. G. Seeley, in South Africa, and has been 



