246 



SCIENCE. 



[Vol. XVI. No. 404 



SCIENCE: 



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Vol. XVI. NEW YORK, October 31, 1890. No. 404. 



CONTENTS: 



The Aim and Future of Natural Origin of Bight or Left Hacded- 



SciENCE 239 I ness. J. Mark Baldwin 247 



The Secor System of Marine • ,r ^ dii- ^ ^. n ..* cao 



Doaf-Mutes. Fhilip 6, Gillett.. . 248 

 Propulsion 244 



.,.- I Chalk from the Niobrara Creta- 

 Notes and News 245 



„ ceous of Kansas 



Letters to the Editor. I 



Fluctuations of Air-Pressure | *• ^- ^'U'ston 249 



H. A. Sazen 246 i Among the Publishers 249 



LETTERS TO THE EDITOR. 



*** Correspondents are requested to be as brief as possible. Tlie writer's name 

 is in all cases required as proof of good faith. 



Tlie editor loillbe glad to publish any queries consonant with the character 

 of the journal. 



On request, twenty'copies of the number containing his communication ivill 

 be furnished free to any correspondent. 



Fluctuations of Air-Pressure. 



It is probable tbat no problem in meteorology has been so puz- 

 zling as the explanation of the diurnal range ot' the barometer. 

 This phenomenon is one of the most constant in raeteorolog.v ; and, 

 in fact, it is so regular near the equator, that Humboldt once said he 

 could tell the time of day in that region by reading bis barometer. 

 On examining a barograph trace made in the tropics, we find a 

 most surprising regularity both in tlie maximum and minimum 

 points of the curve day after day, and also in the amplitude of 

 the range. These conditions, however, do not exist in the tem- 

 perate regions or in those farther noi'th. Here there is superposed 

 upon the diurnal range a mi.\-ed fluctuation, due in the main, if 

 not entirely, to the passage of areas of high and low pressure. 

 These areas are continually passing, and in consequence the 

 diurnal range is masked, or even entirely obliterated on some days. 

 It is known, however, that in general the diurnal range is much 

 increafed on bright, sunshiny days. Gen. Gi-eely found a trace 

 of this range at Fort Conger, 81° 44' north. It is known that the 

 range has two maximum and two minimum point''. The princi- 

 pal maximum occurs all o^ er the globe between 9.30 and 11 A.M., 



and the principal minimum from 2.30 to 4 p.m. The two other 

 points occur approximately at the hours of the same name at 

 night. This remarkable fluctuation is observed at the tops of high 

 mountains, showing that the cause is above the lower atmosphere. 

 The voyages of the " Challenger" and of other vessels have shown 

 that this range is the same over the ocean as on the land, though 

 the water temperature changes very slightly over the former. 



It would be impossible to give in a short space all the explana- 

 tions tbat have been advanced fpr this phenomenon. Changes of 

 temperature and moisture have been appealed to in vain. A po- 

 tential efl'ect has been suggested from the fact that there may be 

 a re-action, as it were, from the air, owing to the increasing heat 

 in the lower strata after sunrise. It is not too much to say that 

 any and all explanations which ascribe this change in jiressure to 

 movements of any kind in the air. to a secondary effect from 

 changes in temperature or moisture, and to any of the forces or 

 agencies usually appealed to in atmospheric movements, have 

 signally and utterly broken down. In 1881 J. Allan Brown (now 

 deceased), after thoroughly examining this question, says {Nature^ 

 April 14. 1881), " If we suppose that the attraction of gravity is 

 not the only attraction which affects the pressure of the atmos- 

 phere, but that this pressure varies through some other attracting^ 

 force, such as an electric attraction of the sun depending upon the 

 varying humidity of the air, and this again depending on its tem- 

 perature, we should find another method of relating the two 

 variations, which does not exist if gravitatien alone is employed," 

 In 1883 the present writer was called upon to give a course of 

 twenty lectures upon meteorology before a class at Fort Myer, Va. 

 In the nineteenth lecture of this course the remarkable similarity 

 betw-eeu the curves of diurnal range in air-pressure and ot the 

 dechnation magnet was pointed out, and a connection between 

 these phenomena, as well as a common origin, were distinctly sug- 

 gested (see Annual Report of the Chief Signal Officer, 1883, p. 

 143). 



There has just come to band an interesting paper on " Diurnal 

 Variation of Terrestrial Magnetism," by Professor A. Shuster 

 {Philosophical Transactions of the Royal Society, London, vol. 

 180, p. 509). I quote from the latter part of the paper, 



"The late Professor Balfour Stewart has suggested that the 

 earth's magnetic force might induce electric currents in the con- 

 vection currents which flow in the upper regions of the atmosphere.. 

 One difficulty of this hypothesis was removed by an experimental 

 investigation, by means of which I proved that the air can be 

 thrown into a sensitive state in which small electro-motive forces 

 will produce sensible electric cui'rents. To bring the air into that 

 sensitive state, it is only necessary to send an electric current 

 through it from some independent source of high potential. It is 

 very likely that the air in the upper regions of our atmosphere is 

 in such a sensitive state; and it is quite possible, therefore, that 

 the induced electric currents suggested by Professor Stewart 

 really exist. In order that electric currents should be induced 

 "which could account for the observed movement of the magnetic- 

 needle, it is only necessary to imagine convection currents in the 

 upper regions from east to west during certain parts of the day, 

 and from west to east at other times. As regards the effect of the 

 sun, we have, indeed, a daily period of the barometer which is 

 probably due to thermal effects. It is curious and suggestive, that 

 the horizontal motion which must accompany the change in 

 pressure is just such as would account for the daily variation of 

 the magnetic needle. In the tropics the principal minimum of 

 the barometer takes place about 3.40 p.m., and the princijml maxi- 

 mum about 9 A.M. According to the theory of waves, there 

 would be a horizontal movement from west to east in the after- 

 noon, and from east to west in the forenoon. The direction of the 

 induced electric currents would be away from the equator in both 

 hemispheres in the afternoon, and towards the equator in the fore- 

 noon. This is exactly the system of currents we have been led to^ 

 starting fi-om the observed magnetic variation. The only diffi- 

 culty I feel in suggesting that the cause of the diurnal variation 

 of the magnetic needle is the diurnal variation of the barometer, 

 lies in the fact that it would oblige us to place the electric cur- 

 rents into the lower regions of the atmosphere, as these only will 

 be much aHecied by the thermal radiation of the sun." 



