NA TURE 



T(,0 



THURSDAY, JUNE 25 



1 90S. 



METEOROLOGY OF INDIAN SEAS. 

 Meteorological Atlas of the Indian Seas and the 

 North Indian Ocean. Prepared chiefly by W. I.. 

 Dallas, under the direction of Dr. Gilbert T. Walker. 

 Pp. viii+.56 charts. (Simla: Published by the 

 Meteorological Department of the Government ot 

 India, 190S.) Price 17s. 6rf. net. 



ALTHOUGH steam has almost entirely superseded 

 sail for the propelling of ships, there neverthe- 

 less appears to be a steadily increasing desire on the 

 part of sailors for accurate infonnation relating to 

 marine meteorology, and recognition of its import- 

 ance in the. interests of navigation. 



A nietcorologicnl atlas, recently published by the 

 Meteorological Department of the Government of 

 India, should therefore be welcomed by seamen who 

 navigate the ocean areas of which it treats. 



This atlas is composed of thirty-six charts, printed 

 in colours, beautifully finished, and accompanied with 

 descriptive remarks. Twelve of these are designed 

 Jo show the average conditions of atmospheric pressure, 

 wind, and sea-surface currents, or " sea currents," 

 as they are termed in this volume. The value of the 

 charts would have been enhanced if results of air- 

 temperature observations had been added. The effect 

 of a prevailing wind upon mean temperature, and 

 the relation between temperature and pressure are 

 problems of interest to students of meteorology. 



Nine charts show the monthly tracks of important 

 cyclones, and cyclonic storms, over the Arabian Sea 

 and Bay of Bengal ; and fifteen charts the conditions 

 prevailing, and the changes taking place, during the 

 existence of typical storms in those areas. 



The charts refer to an area embraced by the 

 parallels of 30° N. and 12° S. lat., and the meridians 

 of 40° E. and 100° E. long. The monthly charts of 

 barometric pressure, wind direction, and force ex- 

 hibit mean results of 8 a.m. observations (local time) 

 for areas of 4° of latitude and 4° of longitude. 



Pressure is shown in red by isobars, but is also 

 given in tenths and hundredths of an inch, in the 

 left-hand upper corner of each 4° square. Wind direc- 

 tion is indicated by an arrow in black, and wind 

 force (by Beaufort scale) by figures at the tail of each 

 arrow, which denote the mean force for the whole of 

 the 4° square to which the arrow refers. The flow of 

 tho currents is shown, in blue, by wavy arrows, and 

 the velocity in nautical miles per 24 hours is given, in 

 most cases by figures attached. 



In the directions for using the pressure data a 

 table is given for the purpose of reducihg readings of 

 the barometer, at any hour of the day, to that of 

 8 a.m. For the sailor this is a useful table. 



A defect in this volume is the absence of any infor- 

 mation with respect to the number of observations 

 upon which the results for each meteorological ele- 

 ment is based. Wind direction, for instance, may be 

 based on 2, 200, or more observations; they may 

 be all for one year, or may be spread over a number 

 NO. 2017, VOL. 78] 



of years ; they mav be recorded by one ship, or by 

 many ships. 



The wind arrows shown on the first twelve charts 

 are said to represent the mean direction of the wind, 

 but the exact meaning of the term " mean direction " 

 is not stated. It may be the prevailing direction that 

 is meant, or it may be the resultant wind. In any 

 case a single wind direction in each square, either for 

 the purposes of scientific investigation or for the 

 uses of the navigator, is inadequate for the repre- 

 sentation of wind distribution, even where the wind's 

 direction is most constant. Information relating to 

 wind frequency, in order to be of value to the sailor, 

 should deal with the percentage of frequency of all 

 winds within definite areas; and for the Indian Ocean, 

 Ba\- of Bengal, and Arabian Sea this is essential. 

 In order to show the march of the south-west mon- 

 soon from east to west at its cominencement, the 

 gradual changes from north-east and north-west 

 monsoons to south-west monsoon, and the reverse, 

 and to exhibit clearly the northern limit of the south- 

 east trade wind, month by month, nothing short ot 

 a complete wind rose will sufiice. 



The wind directions and forces have been, it is 

 stated, extracted as they stood in the ships' log-books. 

 This may be regarded as sufficiently accurate in con- 

 nection with wind direction, as the magnetic variation 

 over the area treated is small, and the deviation of 

 ships' compasses is now usually kept within negligible 

 limits ; but as regards wind forces it is otherwise. 

 The objections in this respect to the method adopted 

 are, however, recognised, and are alluded to as fol- 

 lows : — 



" Thev may represent the ordinary force of the wind, 

 over the square to which they refer, or they may arise 

 as the average of winds of widely varying velocities." 



To meet this difliculty the sailor is referred to the 

 remarks for the month, given in the pages opposite 

 the charts ; but it seems doubtful whether these re- 

 marks will help the sailor much in all cases, as only 

 the most general information is given in this connec- 

 tion. Owing to the absence of information as to the 

 number of observations on which each wind arrow is 

 based, it is not possible to compare the direction and 

 force of the wind in one square with those of another, 

 or to estimate the chances of e.xperiencing any wind 

 other than the mean wind in any particular square. 



The information relating to surface currents has 

 been copied, it is stated, from the " Monthly Current 

 Charts for the Indian Ocean," issued by the Hydro- 

 graphic Department of the .Admiralty. The charts 

 dealing with the track of storms, and those illus- 

 trating typical storms in the Arabian Sea and Bay 

 of Bengal, in different seasons of the year, should 

 prove exceedingly valuable to the sailor, especially the 

 former. 



It is to be regretted that in this work the term 

 "cvclone," used by most meteorologists to define a 

 characteristic distribution of pressure and wind, has 

 been employed instead to express the force of the wind 

 in a tropical revolving storm. The term was originally 

 adopted by Piddington in his " Sailors' Horn Book " 



