366 



NAIURE 



[Fed. 20, 1879 



the European horizon. The discussion of these observa- 

 tions in conjunction with those made in the northern 

 hemisphere, will lead to a much more precise knowledge 

 of the orbit than we hare at present. 



Olbers' Comet of 1815. — In a recent note upon this 

 comet it should have been stated that, acting upon the 

 wish expressed by Olbers at the time, Triesnecker printed 

 his observed differences of right ascension and declina- 

 tion between the comet and comparison-stars in Zeitschrift 

 fur Astronomte, voL ii. The Vienna observations, there- 

 fore, admit of a new reduction, in addition to those pre- 

 viously named. 



DIURNAL OSCILLATIONS OF THE 

 BAROMETER 



IN the "Meteorological Notes" which appeared in 

 Nature, vol. xviii. p. 198,' some interesting results 

 are referred to, which show marked differences in the 

 diurnal variations of the barometer at places quite near 

 to each other, as Greenwich, Kew, Oxford. It is remarked 

 especially that the forenoon maximum in the months of 

 May to July occurs near 9 A.M. at Greenwich, and near 

 8 A.M. at Kew; while at Falmouth and Valentia it is 

 delayed to 11 a.m., or noon, and occurs in June as late as 

 2 P.M. at Helder. 



Having made several investigations relatively to these 

 questions (which I have not been able to publish as yet 

 in detail), I think it may not be without advantage to give 

 at present conclusions relating to the results above 

 noticed. 



It is obvious that it is of the highest importance with 

 relation to the research as to the cause or causes of the 

 remarkable semi-diurnal oscillations of the barometer, 

 that we should have only real variations of atmospheric 

 pressure to deal with, and not instrumental irregularities ; 

 and that, if there is any part of the mean diurnal varia- 

 tions which is due to local causes, we should be able to 

 separate that part from any other which may be due to 

 S«-'*r-"^ '^r cosmic causes. 



When It is ,<»rr,<^rnbered that the range of the mean 

 diurnal variation with us 13 fr«m two to three hundredths 

 of an inch of mercury, and that the epuckc of maximum 

 or minimum may be shifted an hour by a difference of 

 one or two thousandths of an inch, it will be seen how 

 essential it is that the instruments, the observations, and 

 the corrections shall be the best, in order to be sure that 

 we have real variations of atmospheric pressure before 

 us. 



In order to obtain the best possible results, my investi- 

 gations have been limited to observations made in first- 

 class observatories with standard instruments. From 

 observations made during several years at Makerstoun, 

 Dublin, Greenwich, and Brussels, I have sought by the 

 harmonic analysis the functions of sines which represent 

 them most accurately. I give here the equations for the 

 means of the three months in question — May, June, and 

 July. The variation, v, is in ten-thousandths of an inch 

 of mercury ; the origin for each of the four stations M, D, 

 G, and B, is mean midnight {6 = o) : — 



M, f = 56 sin (« + 3550) + 68 sin (2« + 1430) + 21 sin (3^ + 1710) 

 D, 1/ = 51 sin (« + 358°) + 72 sin (?« + 1440) + 25 sin (^6 + 1530) 

 G, i; = 63 sin (« + 3460) + 88 sin (2« + 143-) + 25 sin (3s + 1540) 

 B, z* = 43 sin (« + 3540) + 92 sin (2^ + 1400) + 24 sin (34 + 170°) 



The terms on the right of each equation represent the 

 oscillations, whose superposition completes the whole 

 diurnal variation. We find — 



From the ist term that the epochs of the maximum 

 and minimum were the same within a few minutes at 

 M, D, and B (as shown by the arguments 355°, 358°, and 

 354°), differing at Greenwich from the others by about 

 40m. 



From the 2nd term, that of the semi-diurnal oscillation. 



that the epochs were the same at all the stations within a 

 few minutes. 



From the 3rd term, that they agreed at D and G and at 

 M and B, those for the former being about 23m. different 

 from those for the latter. 



When we consider the coefficients of the different terms, 

 which represent half the ranges of the oscillations, slight 

 differences are found for the ist and 3rd terms; for the 

 2nd the range diminishes regularly as the latitude 

 increases at the rate of o'ooioi inch for each degree of 

 latitude. 



The exact agreement in the epochs of maxima and 

 minima and the regularity of the variation of range with 

 latitude in the semi-diurnal oscillation show that this oscil- 

 lation obeys a general law. Dr. Lamont has supposed that 

 the 1st term, or single oscillation, is due to variation of 

 temperature ; this, I believe, is not the case. When we 

 compare the terms for different seasons of the year, we 

 find that for the sante place the epochs of maximum and 

 minimum may vary twelve hours in the single oscillation,^ 

 while the epochs deduced from the same term for the 

 temperature variations do not differ one hour. Not only 

 so, I have found on the South Indian Ghats that the 

 epochs deduced from the ist term of the barometric 

 equations vary seven hours in ascending 6,000 feet ;. 

 while those shown by the 2nd term are absolutely 

 constant. 



For all these reasons I conclude that the semi-diurnal 

 oscillation of the atmospheric pressure is due to a cosmic 

 cause, independent of local influences, while the single 

 diurnal oscillation shows that part of the solar action 

 which is modified by atmospheric conditions yet to be 

 determined. The results for the four stations just given 

 are a few links in a long chain of facts which tend to 

 prove that the semi-diurnal oscillation of the barometer 

 is due to an action of the sun, which is repeated equally,, 

 twice in each day, like the solar oceanic tide. 



It will be seen, I think, from the results obtained from- 

 the Brussels, Greenwich, Dublin, and Makerstoun obser- 

 vations that the differences noticed at the beginning of this 

 article cannot be allowed to enter as data into the domain 

 of meteorology without much greater study of all the cir- 

 cumstances on which they depend. The facts of atmo- 

 spheric variations are very difficult of explanation, but if 

 we begin to admit results which may be purely instru- 

 mental among these facts explanation will become 

 impossible. 



It is a fact that the true temperature of the mercurial 

 columns has not always been obtained, and when we have 

 to discuss observations with self -registering instruments, 

 many sources of error, including those of temperature on 

 the apparatus itself, have to be cared for.' At stations 

 near the sea, such as Helder, Valentia, and Falmouth, we 

 have also to remember that in the varying height of the 

 partial base of the atmosphere, through the solar oceanic 

 tide, there is a real cause of diurnal barometric oscilla- 

 tion whose amount and epochs should be ascertained 

 and deducted before exact comparisons can be made 

 with observations inland. At the same time I would 

 remark that of the stations here considered Dublin is 

 near the sea, while the others are more or less distant 

 from it. John Allan Broun 



MAROCCO AND THE ATLAS ^ 



THE expedition of which an account is given in this 

 most interesting volume was undertaken by Sir 

 Joseph Hooker and Mr. Ball in the spring of 1871, 

 and lasted a little over two months. Many causes com- 



' The observations here studied at the four observatories are all made by 

 the eye. 



» "Journal of a Tour in Marocco and the Great Atlas." By Joseph 

 Dalton Hooker, K.C.S.L, C.B., Pres. R. S. , Director.of the Royal Gardens, 

 Kew; and John Ball, F.R.S., M.R.LA. With an Appendix, including a 

 sketch of the Geology of Marocco, by George Maw, F.L.S. (London: 

 Macmillan and Co., 1878.) 



