November 28, [918] 



NATURE 



243 



thought. Bui until this has been done the postu- 

 late "that the velocity of light in free space 

 irs the same to .ill observers regardless of the 

 ■ ,• motion ol the soun e and the observer " is 

 <im; that remains a stumbling-block, and the de- 

 tailed mathematical discussion ol the consequences 

 11 h .1 postulate must remain a matter of minor 

 interest. 



In the development of any branch of scientific 

 thought it generally happens that in the lorm 

 finall) assumed the historii order of thought 

 is reversed. The process of analysis of the 

 lex into its constituent elements is re- 

 el !,■ a formal synthesis ol those elements 

 to r< pi idui 1 the original i omplex. '1 o the 

 i it seeking to ge[ a living grasp of 

 the meaning ol science, and not a mere 

 ! abstract parallel to it, it is necessary 

 to go through in his own experience the stages 

 by which the perfected final form of the science 

 has been reached. The teacher and the writer of 

 text-books should therefore seek first of all to 

 give the benefit of his knowledge and matured 

 thought to enable the reader to pass painlessly 

 and naturally through those various phases. 

 When this has been done, the demonstration of 

 unforeseen consequences is legitimate. 



The present author plaees quite lair in 

 the book- those transformation equations for the 

 electric and magnetic intensities which played an 

 absolutely vital part in making the enunciation 

 of the principle possible. One may be permitted 

 to wonder if in so doing he iv not writing with his 

 eye too close to hi- subject, so thai the readei 

 ■I.. i come sufficiently into his field of view. 

 But, this being a matter of common occurrence, 

 it may pass, and the book may in- recommended 

 ccounl of tin- later developments of the 

 theory of relativity, which dwell particularly on 

 those quantities which seem to have a signifi- 

 cance that is not relative. In particular the 

 "action " of a dynamical system is one that has 

 some such absolute meaning if it be true that 

 the motion of a system is to satisfy the principle 

 of least action regardless of the velocity which 

 the observer chooses to assign to himself. The 



invariance of the action seems to be the most 



prehensive summing up of the consequences 



ol the principle of relativity, and at the same time 



opens out the possibility of the generalisation of 



it in the w;i\ thai Einstein has recently achieved. 



This extension of the principle docs not come 

 within the > SCOpe of the book, which com hides 



with a presentation of the four-dimensional vector 



analysis in the form developed In Wilson and 

 I ,e\\ i v 



OUR BOOKSHELF: 

 Die Vegetation des Vol Onsernone [Kanton 



• ■■). By |. Bar. So pp., with coloured 

 phytogeographical map. (Zurich: Rascher and 

 Co., tgi8.) Price 3 francs. 

 I'lils is a very compact description of the vegeta- 

 tion ol a mountain valley-basin south of the Alps, 

 a little 10 the north-west of I. ago Maggiore. The 

 NO. 2561. VOL. 102] 



rainfall ishigh (80 in. to 100 in.), and is nearly all 

 received during the summer. At the same time, 

 the number of rainy days is low and of clear days 

 high, so that a great deal of sunshine is received, 

 I the winter temperatures are relatively high. 

 Thus we have a combination of some of the 

 favourable conditions for vegetation characteristic 

 of .111 "oceanic" climate with some of those char- 

 11 teristic of a continental one, a combination 

 which, together with the great range of altitude 

 (250 m. to 2500 m.) within the area, leads to the 

 irrcnce of a very wide range ol vegetation and 

 a very large number of species. The vastly greater 

 proportion of the whole area of 113 sq. km. is 

 covered with trees and shrubs. The general alti- 

 tudinal forest zonation on the northern exposures 

 is chestnut, beech, silver lir, spruce, and larch, 

 with the addition ol extensive birch woods and 

 more local lime and grey alder woods according 

 to the soil conditions. Besides these there are 

 extensive scrub associations of hazel, chestnut, 

 birch, beech, and oak, which play an important 

 part in the economy of the valley as pasture 

 for goats, besides unpastured scrub of willow, 

 alpine alder, alpine rose, juniper, etc., "heaths" 

 of heather, broom, and bilberry, and numerous 

 tvpes of grassland. Many of the associations are 

 almost identical with common British types. 

 Above these there is a wide selection of alpine 

 types, and at the other end of the scale an asso- 

 ciation of the Mediterranean Cistus salviifolius. 

 The memoir is accompanied by an excellent vege- 

 tation map, in which the distribution of the 

 dominant trees and shrubs is depicted by means 

 of coloured symbols. 



Food Gardening: For Beginners and Experts. By 

 H. Valentine Davis. Second edition, revised 

 and enlarged. Pp. viii+133. (London: G. Bell 

 and Sons, Ltd., 1918.) Price is. net. 

 This handbook will prove useful to the allotment- 

 holder, as it sets forth clearly and fully the details 

 of cultivation of the commoner vegetables. The 

 first part of the book, however, contains several 

 instructions that are misleading and may cause 

 difficulty. It is contrary to all accepted usage to 

 grow root crops and onions on soil which has not- 

 been dug or even forked over since the removal 

 of the previous crop. The plan may succeed on 

 lig-ht land, but on heavy, sticky soil such a pro- 

 cedure would probably court failure. For many 

 districts June is very' late for planting maincrop 

 potatoes, and parsnip-sowing should not be de- 

 ferred until April. It is also beside the mark to 

 recommend that green peas should be eaten raw, 

 and to suggest that discarded woollen garments 

 and leather articles should be used for manurial 

 purposes. On the other hand, the details of work- 

 ing are usually well explained, and the' calendar 

 of operations affords a useful guide to the approxi- 

 mate times for carrying out the more important 

 pieces of work. The second part, dealing with 

 such crops as tomatoes and celery, is very lucid, 

 and the hints on the winter storing of vegetables 

 and on the destruction of diseases and pests are 

 valuable and practical. 



