378 



NATURE 



\August 1 6, 1883 



month of November there was a very prominent simultaneous 

 dov\nward movement at the three stations, a movement which 

 must have bent the curves very considerably out of the shape 

 they would have taken had it not occurred, and it is allowable 

 to suppose that the proper minimum in the Belgaum and Bombay 

 curves corresponding to b" of the Zanzibar curve took place in 

 the month of October, 1881, that is to say, after the normal 

 interval of five months, but was masked by the greater minimum 

 in November, due to the simultaneous movement. Then again 

 in the case of the maximum movement c, c', and c", the period 

 between c and c" is, if five months be assumed to be the 

 normal, quite regular ; but between c' and c" it is only four 

 months, that is, one month shorter than usual. A reference to 

 the dotted lines shows that in the month of January, 1882, all 

 three curves were upheaved by a simultaneous movement, while 

 in the follow ing month they were all three depressed simul- 

 taneously. By the co-operation of these two simultaneous 

 movements, the maximum c' was apparently quickened in its 

 course by one month, and hence the irregularity. Again, with 

 regard to the double oscillation D, d', and d" (i and 2) in the 

 Zanzibar curve the first downward bend d"j is greater than the 

 second d" 2 ; but in the Belgaum curve they are very nearly 

 equal, and in the Bombay curve tbe first is even less than the 

 second. On glancing down at the Zanzibar curve for the month 

 of April, it is observable that an upward movement took place 

 then ; and if it be supposed that the upward impulse was felt 

 at all the three stations simultaneously, but that this impulse was 

 not so great at Bombay and Belgaum as the downward impulse 

 due to the travelling movement coming from Zanzil ar, then the 

 actual effect at those two stations w ould be the resultant of the 

 two impulses, that is to say, a downward movement of less 

 amplitude than would have occurred had there been no simul- 

 taneous movement in that month. 



The apparent acceleration of the movement A, A', and a" is 

 susceptible of a similar explanation, though not quite so satis- 

 factorily, and it may perhaps be admissible to reserve for it an 

 explanation which will piesent itself hereafter. 



The existence of these simultaneous movement-; seems not 

 ■only to afford an explanation in great part of the irregularities 

 observable in the eastward transmission of the travelling move- 

 ments, but alo to clear away an objection that was brought 

 forward by Mr. E. Douglas Archibald to the acceptance as an 

 established theory of the eastward movement of abnormal 

 variations. He asked [vide Nature, vol. xxiii. p. 400) "Why 

 tbe barometric waves should commence on onr meridian rather 

 than one another." Now it is very noticeable (if a reference be 

 made to the curves) that all the marked features of the curves — 

 those features that are transmitted eastwards— occur in months 

 when there are simultaneous movements at all the three stations, 

 that in fact the simultaneous movements are the initial ones. 

 And in the light of this fact the answer to Mr. Archibald's 

 objection is that they do not commence on one meridian rather 

 than on another, but (so far at any rate as the three stations under 

 consideration are concerned) on all meridians simultaneously. 

 But it is likely enough that thty may be greater on one par- 

 ticular meridian, or at one particular point on that meridian, 

 than on those on either side of, or about it, that in fact they 

 result from a slight heaping up or withdrawing of the atmo- 

 sphere over, or from, one part of the earth's surface, in which 

 ■case the heap, or depression, will have its greatest altitude or 

 depth at one particular place, but of course will be felt over a 

 more or less on iderable area around that place, and the degree 

 in which it will be felt will be less as the length of the radius 

 from the centre is increased. And that this is not altogether a 

 fancilul idea is apparent on a reference I eing marie to the 

 smoc thed curves, « hen it will be observed, for instance, that in 

 July, 1880, and also in June, 1881, the upward movements 

 were much greater at Bombay, the most northern of the three 

 stations, than at Belgaum, a more southerly one ; and at this, 

 again, they were much greater than at Zanzibar, the most 

 southern. 



Mr. Archibald brings forward another objection. He asks : 

 "If, as Mr. Chambers thinks, the waves of pressure travel 

 slowly round the earth, why thty do not reappear at the place 

 where they started, after an interval of about one year and 

 eight months (calculated from the lags given in Mr. Chambers's 

 paper). At present there does not appear to be the slightest 

 eviderce that they reappear at all, and if they do not, when and 

 wheie do they disappear?" One answer to this question is that 

 they n ust, in the course of their eastward journey, get com- 



pletely masked by other simultaneous movements of the atmo- 

 sphere that are constantly taking place. Another answer to this 

 question, and the one not requiring the supposition of the simul- 

 taneous movements, is that, as the travelling u aves get further 

 away from the place of their origin, and consequently widen 

 out, their amplitude gets constantly less, until at last, like the 

 waves caused by dropping a stone in a pond, they become im- 

 perceptible. If it were posihle to eliminate the effect of the 

 simultaneous movements, and examine only the curve produced 

 by the travelling wave*, one might then see this gradual decrease 

 in their amplitude as they proceeded along their journey. It is 

 impossible, however, at present to separate the effects of the 

 two movements. An alternative method, however, to eliminating 

 the effect of the fir>t-mentioned movements is to pick out a 

 period during which they were small or imperceptible. If such 

 a period can be found, it will then doubtless be possible in 

 some degree to trace the comparatively undisturbed action of the 

 travelling movements. Such a period occurred from March to 

 Augu-t, 1882, during which time the simultaneous abnormal 

 movements were not easily traceable. And it is then seen how 

 the amplitude of the double oscillation d'' (i and 2) of the 

 Zanzibar curve has diminished at d' and D (1 and 2) of tbe 

 Belgaum and Bombay curves. 



Another question which may be raised with regard to the 

 matter, and a question which is not so easily answered is, why 

 these waves should travel in an easterly direction and not in a 

 westerly ? It would be imagined that they should be trans- 

 mitted equally in both directions, or if they are transmitted in 

 one direction rather than in the other, it should have been a 

 westerly one ; in which case their motion might have been ac- 

 counted for readily enough by supposing the atmosphere to lay 

 behind in equatorial regions in a westerly direction due to the 

 influx of air of a lo^er velocity from the polar regions; and 

 perhaps also by supposing the sun to exercise an influence in the 

 matter. The fact is, however, that the motion is in the same 

 direction as, and ahead of, the earth's rotation. It would be 

 interesting, however, to see if there is any evidence of a west- 

 ward motion, and referring to the curves with this object in 

 view, such evidence is perhaps discoverable. For instance, the 

 simultaneous movement in July, 18S0, causes a very marked 

 upward bend of the Bombay and Belgaum curves; if, then, 

 there is any motion westwards, this upward bend should make 

 its appearance in some succeeding month in the Zanzibar curve ; 

 and, as a matter of fact, there is an upward movement shown 

 ly the dotted line in the month of September. It is difficult to 

 find many instances of this westward transmission, owing to the 

 somewhat intricate mixture of movements presented by the 

 curves ; but the following instances may be adduced as lending 

 some support to the hypothesis : there is a simultaneous down- 

 ward movement in May, 1880, and there is an independent 

 downward movement at Zanzibar in the month of August in the 

 same year, that is, three months later ; there is again tbe instance 

 already cited of the simultaneous upward movement of July, 

 1S80, recurring at Zanzibar in September, that is, two months 

 later. It may be that the excessive downward movement at 

 Zanzibar in the month of March, 1S81, was in part due to the 

 recurrence there of the simultaneous movement which occurred, 

 especially developed at Bombay, in January of the same year ; 

 that is, two months before. Again, the very large downward 

 movement at Zanzibar in October and November, 1881, may 

 have been in part due to the arrival there from Bombay and 

 Belgaum of the wave produced by the simultaneous downward 

 movement which occurred in August, two and a half months 

 earlier. And lastly, two instances less difficult to trace, owing 

 to the absence of any marked simultaneous movements during 

 the period of their occurrence, are the upward and downward 

 movements at Zanzibar of the months April and May, 1S82, 

 which may be regarded as due to the arrival there from the west 

 of India of the waves resulting from the simultaneous impulses 

 received at all stations in January and February of the same 

 year ; that is, at periods of three n onths for each. 



And here may be given the explanation previously referred to 

 of the apparent acceleration in the rate of movement of the 

 wave A, a', and a". It is possible that A and a' may not be 

 due to the arrival at Bombay and Belgaum of the maximum a", 

 but of the wave caused by the simultaneous movement which 

 occurred in July six months before, a period much nearer the 

 normal than are the periods th r ee and three and a half months; 

 in which case the maximum a" would be due to the arrival at 

 Zanzibar from Bombay and Belgaum of the wave caused by the 



