January 21, 1892] 



NATURE 



277 



of a moderate gale from the north-westward, and much 

 thunder and lightning occurred. Unsettled weather con- 

 tinued from the 21st to the 24th, and from this day a 

 storm area can be clearly traced travelling to the west- 

 ward. The cyclone reached its greatest violence on 

 June 2 and 3, when the barometer is reported as reading 

 27-86 inches in close proximity to the centre of the dis- 

 turbance. A hurricane occurred at Obokh during the 

 evening of the 3rd, and it was reported that all the 

 houses but one had been blown down, and trees had 

 been uprooted. The position of the storm area is not 

 only marked throughout its passage across the Arabian 

 Sea by the cyclonic circulation of the winds, but also by 

 the rain area which accompanied the disturbance ; the 

 rate of progress of the storm from May 24 to June 3 was 

 rather less than seven miles an hour. 



The second cyclone which is shown by the charts ap- 

 pears to have originated not far distant from Ceylon at 

 the commencement of June, and on the 4th a strong 

 south-westerly gale was blowing on the equator in the 

 longitude of 76° E. This storm can be traced for the next 

 ten days, during which time it passed to the northward and 

 westward towards the entrance of the Persian Gulf. The 

 weather was very disturbed over nearly the whole of the 

 Arabian Sea from the 9th to the 13th, and the area of the 

 storm was much larger than in the case of the Aden 

 cyclone, and gales were experienced from the coast of 

 Africa to that of India, extending over a distance of about 

 1500 miles. The synchronous weather charts for the last 

 few days of the discussion, after the cyclonic disturbances 

 had passed away, show that the south-west monsoon had 

 extended over the whole of the Arabian Sea, whereas in 

 the middle of May it was limited chiefly to the western 

 side. 



Each daily chart contains the observations from several 

 ships in the Red Sea, where the wind direction and other 

 elements of the weather are very instructive. The 

 southerly march of the northerly or north-westerly wind, 

 which throughout the whole period prevails over the 

 northern portion of the Sea, and the gradual backing 

 down of the southerly winds in the southern portion of the 

 Sea are well shown. The northerly wind in the northern 

 portion of the Red Sea often attains the force of a gale, 

 but there is no instance in the charts of the southerly 

 winds attaining gale force. The air temperature is 

 generally higher in the Red Sea than over the more open 

 water in the Arabian Sea, the reading of the thermometer 

 commonly reaching 90^, and on June 14 the temperature 

 at 10 o'clock in the morning was 102° over the open sea, 

 nearly abreast of Musawwd. The charts show many other 

 points of interest, among these the flow of the current 

 under the influence of disturbed weather as well as when 

 the sea is comparatively quiet, and doubtless the volume 

 will throw some additional light on the winds and weather 

 in this part of the world, where at present the meteoro- 

 logical changes are not too well understood. 



ONVANDER WAALS'S ISOTHERMAL 

 EQUATION. 



IN reply to Prof. Tait's criticism (Nature, December 

 31, 1891, p. 199) of my paper (December 17, p. 152), 

 I wish to say that I certainly do not consider Van der 

 Waals's b as an absolute constant. Perhaps it may be 

 interesting to show how the limits of its variability can 

 be determined. 



Leaving aside the question of the attractive forces, 

 which probably has been sufificiently elucidated in the 

 course of this discussion in the columns of Nature, and 

 considering gases as aggregations of elastic spheres, then 

 in the formula — 



p^{v - xb^ = i2w«2 (I) 



X can be proved to be equal to 4 for large volumes and 

 sma// pressures. 



NO. I 160, VOL. 45] 



Again, in the case of extremely large pressures, when 

 the volume is nearly reduced to the' smallest possible 

 dimensions, it is easy to see that a formula — 



py{v - fLb^) = JA. . Itnu"- (2) 



must hold good, where \i.b^ = 3 ^2/77 . ^1 = i "35 . . <Ji re- 

 presents the space in which the spherical molecules can be 

 inclosed when they are motionless, and X is a certain 

 numerical coefficient whose determination might present 

 some interest, and perhaps is not beyond the scope of 

 mathematical analysis. (For one-dimensional motion 

 X = I.) Be this as it may, putting (2) in the form— 



,[»-(, 



M^ 



M^i 



)./x^,1= Itmu'' 



(3) 



it is clear that in this case x approaches the value 

 M = I-35- 



Now surely, for /«/^r/«i?rt'/<a!/^ volumes and pressures, jr<^i 

 cannot be considered as a constant ; still, along the large 

 range of these pressures, the correction required must be 

 called relatively slight, and the more so as it is beyond 

 doubt that a considerable part of the change from 4 to 

 I "35 takes place near those extreme pressures where, 

 according to (3), x may be very variable. Whether at 

 the critical volume this coefficient has undergone already 

 a practically important change from its original value, 

 4, seems to me a question which cannot easily be 

 answered by purely theoretical considerations. 



In my opinion, in all cases except in that of large 

 volumes the formula (i) is preferable to a formula 



p^-i^-^^^^-^^^^ 



(4) 



even if the numerical value of <r could be exactly calcu- 

 lated ; therefore the question at issue does not simply 

 turn on the introduction or rejection of terms of the order 

 ^'^jv'^, and it was looking at the matter from this point of 

 view that in my paper I once called a formula of the 

 form (i) the true o»e as distinguished from a formula of 

 the form (4), and not from any formula given by Prof. 

 Tait. Certainly, none of the isothermal equations given 

 by different authors can be named true in the sense 

 of representing with absolute exactness the conduct of 

 real gases ; and of course, when more constants are in- 

 troduced in these equations than are contained in that of 

 Van der Waals, a better approximation to the conduct of 

 these gases may be reached. 



In conclusion, I beg to add a few words about Prof. 

 Tait's third remark. It seems to me that he has no right 

 to identify the process of putting arbitrarily y = jS with 

 that of calculating the correction indicated by Prof. 

 Lorentz. D. I. KORTEWEG. 



Amsterdam, January 6. 



NOTES. 

 Several scientific meetings have been postponed in con- 

 sequence of the death of the Duke of Clarence. Prof. W. E. 

 Ayrton, F. R.S., was to have delivered his inaugural address, as 

 President of the Institution of Electrical Engineers, on January 14. 

 It will be delivered at a meeting of the Institution on January 

 28. The annual general meeting of the Royal Meteorological 

 Society, fixed for the 20th, will be held on the 27th, when the 

 President, Mr. Baldwin Latham, will deliver an address on 

 " Evaporation and Condensation." The annual meeting of the 

 Entomological Society is also adjourned from the 20th to the 

 27th. 



The forty-fifth annual general meeting of the Institution of 

 Mechanical Engineers will be held on Thursday and Friday 

 evenings, February 4 and 5, at 25 Great George Street, West- 



