
DIURNAL OSCILLATIONS OF THE BAROMETER, 399 
the geographical distribution of the amounts and times of the oscillations. It 
is to meet this desideratum that we have collected during the past ten years, 
as opportunity offered, the following data :— 
1st. Observations from 335 places in all parts of the globe, showing the 
mean amplitude of the oscillation from the morning maximum to the afternoon 
minimum for each month and the year. These are given in Table I., with the 
hours of observation and the number of years for which the averages have been 
taken. 
2d. Mean horary states of the barometer from 81 places, and bi-horary 
means for 5 places, as indicated in Table I. by “Hourly” in column of “Hours 
of Obs.” 
It is believed that these include nearly all the observations at present avail- 
able for such an inquiry. For much valuable assistance in this inquiry, I 
have to express my best thanks to Dr Hann, Vienna; Dr Buys BAttot, 
Utrecht ; Professor Moun, Christiania; M. Mari& Davy, Paris; Dr Gustavus 
Hinricus, Iowa City; and Mr Hartnup, Liverpool. 
Of the four daily oscillations, the most important, in relation to the daily 
march of temperature and vapour, and as respects amplitude in nearly all cases, 
is the oscillation from the morning maximum to the afternoon minimum. For- 
tunately, this also is the one oscillation regarding which meteorological observa- 
tions supply the fullest data—mean monthly and annual results having been 
obtained, as just stated, for 335 places in different parts of the globe. Since 
sufficient materials are thus available for giving a first approximate representa- 
tion of the amount of this oscillation over the globe from month to month, it is 
proposed in the meantime to limit the inquiry exclusively to this oscillation. 
Time of occurrence of the A.M. maximum.—From the hourly observations it 
is seen to occur in January, from 9 to 10 in tropical and sub-tropical regions, 
as far as 50° lat. N.; in higher latitudes the time varies from 7 at Bogoslovsk 
in Siberia (60° lat.), to 11 at Helder in the Netherlands, and noon at Valentia in 
the south-west of Ireland, both places being almost surrounded by the sea. In 
July it occurs from 9 to 10 at stations only so far as about 40° lat. In higher 
latitudes the times vary, being 8 or even 7 at many continental places, and as 
late as 11 at some places near the sea, and at noon at Helder and Valentia. 
Time of Occurrence of the P.M. Minimum.—In January it occurs gene- 
rally from 3 to 4; but there are many exceptions north of lat. 40°, where it 
occurs at 2, and, in one or two cases, as early as 1 o’clock. It is quite different 
in July, when the time from 3 to 4 is pretty regularly observed as far as about 
35° lat.; but in higher latitudes the times are 5 and 6, the latter hour being 
observed at the more strictly continental stations. During this season at 
Melbourne and Hobart Town in the southern hemisphere, the hour is 2 P.m., 
being in this respect similar to the winter of the northern hemisphere. 
