432 



NATURE 



\March i^;'i87g 



Dr, Billings very sensibly suggests that all medical men 

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 the Index and taking care that a copy of every book, 

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 to the editors. This recommendation we heartily endorse, 

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LETTERS TO' THE EDITOR 



\The Editor dtes not hold himself responsible for opinions expressed 

 ■ by his correspondents. Neither can he undertake to return, or 

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 notice is taken of anonymous communications. 



[The Editor urgently requests correspondents to keep their letters as 

 short as possible. The pressure on his space is so great that it 

 is impossible otherwise to ensure the appearance even of com- 

 fnunications containing interesting and novel facts.J 



Th« Tides at Chepstow and Fundy 



In Nature, vol. xix. p. 363, Mr, Moseley, of Exeter College, 

 Oxford, quotes a passage from Lyell's Principles, to the effect 

 that the tides at Chepstow have a range of 72 feet. 



This statement is continued in the last edition (1875), in 

 Chap. XX., on Tides and Currents, p. 492; and the tidal 

 range in the Bay of Fundy is set down at 70 feet (p. 564). I 

 do not believe either of the statements. 



The Admiralty Tide Tables give the following : — 



Spring Range. Neap Range. 

 Feet. Feet. 



1. Chepstow 38 28-5 



2. Bay of Fundy (Noel Bay) ... 50*5 43*5 



I may add that Lyell's whole treatment of the subject of the 

 tides is loose and inaccurate. Thus, for example, he says 

 (p. 491): " In any given hne of coast the tides are greatest in 

 narrow channels, bays, and estuaries, and least in the inter- 

 vening tracts where the land is prominent." 



He then proceeds to illustrate this proposition by giving the 

 ranges of tide from the mouth of the Thames to Flamborough 

 Head (including, of course, the well-known tidal node of mini- 

 fnum range off Yarmouth) ; but he is utterly unconscious that 

 these ranges depend altogether on the tidal motion of the water, 

 and have no relation whatever to the form of the coast. 



Trinity College, Dublin, March 3 Saml. Haughton 



Magnetic Storms 



It is surprising that an accomplished telegraphist like Mr. 

 Mance (Nature, vol. xix. p. 409) should not see the necessity 

 andadvanta^^e of expressing earth-currents in webers. It is pre- 

 cisely because every one can, if he likes, appreciate the magnitude 

 of an earth-current so expressed, and no one but himself can do 

 so if Mr. Mance's plan were adopted, that I advocate the weber, 

 or rather, its more convenient sub-multiple, the milliweber. A 

 milliweber is the current produced by one Daniell's cell (strictly 

 one volt), through 1,000 ohms. Currents can be reduced to this 

 unit from any galvanometer. The tangent galvanometer is, 

 perhaps, the simplest to use — it is that which we employ in 

 England. Supposing for simplicity that your constant, viz., 

 one Daniell cell through 1,000 ohms (including cell and galvano- 

 meter) gives 45°, then the tangent of any other reading will give 

 you the current in milliwebers. Then, knowing the resistance of 

 your circuit and its geographical position, you have all the data 

 necessary to determine the elements of earth-currents. 



I will act on Mr. Mance's suggestion, and bring the matter 

 before the Society of Telegraph Engineers v.dth a view of 

 organising a systematic mode of observation in different parts of 

 the world, W, H. Preece 



Wimbledon, March 8 



Table Sho'cving Difference of Mean Pressures at Calcutta as Cotn- 

 pared with Bombay for the whole Year and the Swnmer 

 Months, the Sign -i- Indicating an Excess at Calcutta, and 

 — a Defect ; and Character of the Rainfall in the North- 

 West Provinces and Behar. 



Atmospheric Pressure and Solar Heat 



May I be permitted to supplement the table given by Mr. 

 Allan Broun in Nature, vol. xix. p. 7, by the following figures 

 for Calcutta. The pressure anomaly at Bombay for each year, 

 as given by Mr. Broun, is here compared with Calcutta, and the 

 table is extended down to 1877 : — 



Since from 1847 to 1852 there are frequent blanks in the 

 registers, sometimes exceeding ten days in length, the figures are 

 of somewhat doubtful value. 



It will be seen on examination that the decennial period is 

 nearly as distinctly marked at Calcutta as at Bombay, though the 

 minor fluctuations are more frequent, and that, as Mr. Chambers 

 supposed, the amplitude of the anomaly, like that of the diurnal 

 and annual variation, is greater at Calcutta than at Bombay. 



Mr. Broun considers the discovery of the decennial period of 

 barometric pressure to be one of great importance, as forming a 

 link in the chain of evidence which connects the variations of 

 rainfall with those of the sun's heat. As an illustration of the 

 way in which the variations of pressure influence the distribution 

 of rainfall, I have entered a word descriptive of the character of 

 the rainfall of each year in the North- West Provinces and Behar. 

 In the great majority of cases a relatively low pressure at Cal- 

 cutta, especially during the summer months, April to September, 

 means defective rainfall over the valley of the Ganges, which is 

 watered by easterly winds from the Bay of Bengal, and a rela- 

 tively high pressure at Calcutta means excessive rainfall. Had 

 the station with which Calcutta is compared been situated seven 

 or eight degrees to the north of Bombay, the rule would j^robably^ 

 have been without exception. The influence of what he calls | 

 the "relative barometric anomalies" upon the distribution of 

 rainfall in India has been pointed out by Mr. Blanford on more , 

 than one occasion since 1868, and the examples here given will ! 

 illustrate and enforce this point. j 



Mr. Broun's remarks regarding the relations between the range j 

 of the monthly means of barometric pressure and that of the ' 

 monthly mean temperature, are interesting as confirming tlie 

 views put forward by General Strachey, as long ago as 1850, i'.; 

 an unpublished work on the physical geography of the Himala} 

 and the neighbouring countries. The range of the monthly mea 

 pressure at Calcutta is 0*488 inch, and that of the monthly me. 

 temperature i8°-6 F. The range for one degree is therefc 

 •026 inch, a figure which differs little from those for Madras, 

 Bombay, and Trevandrum. S. A. Hill 



Allahabad, December 7, 1878 tj . 



