568 



NATURE 



[October 8, 1908 



line of the essential points of plant anatomy and 

 histology, may be regarded as preparatory to the more 

 serious systematic study of the vegetable kingdom con- 

 tained in section iii. As might be expected, Prof. 

 Bokorny has adopted the Englerian scheme of classi- 

 f:cation, but reference is made to other systems, the 

 Linnean system being considered, in a special chapter, 

 at greater length perhaps than is desirable at the 

 present day. The greater part of the section is de- 

 voted to the principal orders of phanerogams; and in 

 liis emphasis of well-chosen points of taxonomic im- 

 portance and frequent references to plants of economic 

 and biological interest, the author has produced a 

 very clear and readable exposition of a branch of 

 botany notoriously dilificult to deal with in a manner 

 which shall arouse and, more especially, sustain the 

 interest of the student. 



The question of general morphology is dealt witli in 

 the first section of part ii. Both the stage at which 

 this important branch of botany is considered and the 

 relatively small amount of space allotted to it— some 

 twenty pages largely occupied by illustrations—would 

 probably meet with criticism at the hands of most 

 English botanists, and the same may be said with re- 

 gard to the comparatively little attention paid to 

 anatomy. But the principal features of part ii. aie 

 the sections dealing with physiology and ecology. 

 Ill the latter section the author prefers the primary 

 title of " Biologie der Pflanzen "; the various factors 

 influencing plant life are first considered in some detail, 

 and the actual studies of typical formations are con- 

 cerned with the vegetation of the earth as a whole 

 rather than with a detailed consideration of more re- 

 stricted areas, a method more generally adopted in this 

 country. 



A feature of the book is the wealth of illustrations 

 with which it is provided. Most of them are familiar 

 friends, but they are drawn from very varied sources, 

 and the inclusion of many of them is a further ex- 

 ample of the freshness of conception which has been 

 already commented upon as characterising these 

 volumes. 



OVR BOOK SHELF. 

 Dosmrhigcn i Norge. Bv Prof. H. Mohn. Pp -6 



(Chnstiama : Jacob Dybwad, 190S.) ' 



In a country which stretches, as Dr. Mohn 

 reinmds us, to the 71st degree of north lati- 

 tude, the tunes of sunrise and sunset, with 

 the accompanymg phenomena of twilight have 

 a wider significance than with us. 'There 

 the calendar has to be consulted to find the dav 

 when the sun will first appear above the hori/on 

 while the amount of light received when the sun is 

 a definite distance below the horizon has a distinct 

 economic value. Even the azimuth at which the 

 sun will rise or set is not altogether a neglicrible 

 quantity. Considerations of this kind have led Prof 

 Mohn to submit the question of twilight to a verv 

 close investigation, and to furnish tables which will 

 enable an inhabitant of these northern regions to 

 gauge very accurately how much direct or 'reflected 

 sun ight he may expect. No doubt Prof. Mohn is 

 well advised from a practical point of view, but in 

 some respects his tables seem to aim at a greater 

 NO. 2032, VOL. 7S] 



degree of accuracy than can be of service. In such 

 questions as the effect of temperature on refrac- 

 tion, or the amount of reflected light, the variables 

 arising from clouds and state of the skv generally 

 would upset the nicety of the calculations. But so 

 far as the convenience of the tables is concerned, 

 and the thoroughness with which the theory is pre- 

 sented, there is nothing left to be desired, and it is not 

 surprising if those who have not lived in a country 

 where the economy of the winter light is a matter 

 of importance fail to appreciate the necessity of this 

 accurac)'. 



Prof. Mohn recognises four distinct steps in the 

 approach of night or dawn, (i) The true time of 

 geometrical sunrise or sunset when the sun's 

 upper limb is on the astronomical horizon, or 

 Z, =90°+)' — T + p, where p is the refraction, r the sun's 

 radius, and tt the parallax. (2) The beginning and. 

 end of the gloaming (Skumringens Ende), when the 

 sun's centre is 4° below the horizon. In clear weather 

 in Norway, indoor work is possible under these con- 

 ditions. Bright stars begin to appear in the skv. 

 Sirius is visible when the sun is three degrees below 

 the horizon. (3) It is more difficult to understand 

 what is meant by the end of twilight. It is the time 

 when daylight decreases most rapidly, and is described 

 as the time when, in a clear sky, print can be read with 

 dilficulty if the light from the illuminated part of the 

 sky is allowed to fall on the page, or when some kinds 

 of outside work may be carried on. As a matter of 

 computation, the time is decided by increasing the 

 zenith distance of the sun, given in the first case by 

 small angles depending on the atmospheric refraction, 

 making the sun's zenith distance about 98°. (4) The 

 last stage is that of complete night, or the time when 

 the earth's atmosphere receives no light from the 

 sun. The sun is then about 17° below the horizon. 

 This scheme is a great practical advance on the 

 method adopted in this country, where an arbitrary 

 zenith distance of 108° is accepted as that at which 

 night begins or ends. Tables are given for extending 

 the calculations to other latitudes, and would make 

 them ■ available in the Shetland Isles and North 

 Scotland. 



Maryland Weather Service. \'o\. ii. 1907. Pp. 515; 

 illustrated. (Baltimore : The Johns Hopkins Press, 

 1907.) 



This volume contains a report on the climate and 

 weather of Baltimore and vicinity, prepared bv Dr. 

 O. L. Fassig under the direction of Prof. W. L. 

 Moore, chief of the United States Weather Bureau; 

 it is based on observations of the latter service since 

 1871, supplemented by all available records, both 

 public and private, extending over a period of nearly 

 a century. Meteorologists owe a debt of gratitude 

 to the board of control of the Maryland Weather 

 Service, and to Dr. Fassig especially, for one of the 

 most complete and valuable meteorological discus- 

 sions extant. Part i., which occupies more than half 

 the volume, deals with climatic factors, each element 

 being considered, so far as possible, with reference tn 

 its annual and diurnal periods and its variability; the 

 statistical tables are supplemented by the usual range 

 diagrams and also by isopleths, the principle of which 

 was devised many years ago by M. Leon Lalanne. 

 .Although not frequently employed, the latter method 

 exhibits in a concise and intelligent wav the suc- 

 cessive changes throughout the year. The value of 

 this section of the work is much enhanced by careful 

 discussion of the results obtained and of the inter- 

 action of the various elements, by references to the 

 present state of our knowledge and to generally ac- 

 cepted theories. 



