A Contribution to the Study of the Transmission Eastwards Round the Globe of Barometric Abnormal Movements ¹

Abstract

II.

WHEN the fact of these simultaneous movements is recognised, the irregularities in the transmission eastwards of the abnormal movements can be in great part explained. For instance, taking the movements B″ of the Zanzibar curve, it is found to recur at B′ and B in the Belgaum and Bombay curves after an interval of six months, that is to say, about one month longer than the average, and is moreover of much greater magnitude in these curves than at Zanzibar. But it is noticeable that in the month of November there was a very prominent simultaneous downward 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 following month they were all three depressed simultaneously. 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″ (1 and 2) in the Zanzibar curve the first downward bend D″₁ is greater than the second D″₂; but in the Belgaum curve they are very nearly equal, and in the Bombay curve the 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 Zanzibar, then the actual effect at those two stations would 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 simultaneous movement in that month.