1064 
were selected by Lockyer as the representatives of both types of 
the barometric periodical oscillation of 3.5 years’) Moreover the 
amplitude in Australia is much greater than elsewhere. 
The variations of the airpressure in North Australia from the 
normal value are shown in curve I of the plate, by means of the 
six-monthly deviations of the barometric pressure at Port-Darwin, 
marked monthly on the piate in such a way that, for instance, the 
deviation in the period January-June (in relation to the normal value 
ir. the same months) is drawn on the ft of April. Beginning with 
1899 the base value has changed, apparently because something 
has been altered in the barometer or its position. 
The curve shows some very regular series of waves, namely from 
1878 till 1881, from 1885 till 1891, from 1896 till 1904. whereas 
in 1911 a new series seems to have begun. The maxima and minima 
are characteristic of fired seasons, they develop themselves namely in 
the first and last months of the year. 
Minima. Maxima. 
Dec. 1878 1 Febr. 1881 
Oct. 1886 1 Sept. 1885 
Febr. 1890 1 Jan. .1889 
Febr. 1898 1.-Oet.. . 1891 
Sept. 1900 1 Nov. 1896 
1 
1 
1 
nn jk 
Febr. 1904 Febr. 1900 
Dec. 1902 
Mareh 1912 
It is further evident, that the time which elapses from minimum 
to maximum is one year, from maximum to minimum two years. The 
period is exactly 3 years. 
The curve so closely resembles this schematic interpretation, that 
it may be represented by the schematic broken line IV of the plate. 
From 1878 it has been traced backward by means of the barometric 
observations made at Adelaide. For simplicity’s sake tne maxima 
and minima have been drawn on the 1st of January. 
These regular periods are. particularly adapted to forecast the 
airpressure a considerable time in advance. Disturbed periods are 
lying between them however, in which the curve makes the impres- 
sion that there has been no development of the maximum for some 
years. The certainty with which the barometric variations may be 
predicted would decrease greatly, if the epochs at which these disturbed 
1) Solar Physics Committee. Monthly mean values of barometric pressure. 
