39 
January, and the minimum in July, precisely as in the British 
Islands, and therefore agreeing- with the law of disturbance 
announced by the Author in a former Paper. There is a 
small maximum in March, and a rather sudden and consider- 
able increase of disturbance in October. The mean annual 
amount of oscillation is 31-686 inches, and the mean annual 
number of oscillations 156-9. The mean range and mean 
duration of oscillation are both greatest in the winter, and 
least in the summer months. The greatest range occurs in 
January, but the greatest duration of oscillation is in Novem- 
ber. It is remarkable, too, that the range is also greater in 
November than in any other month except January. These 
results for November are probably due to the great barometric 
wave which Mr. Birt some years ago pointed out as fre- 
quently occurring on or about this month. 
In England the mean duration of oscillation is greater in 
the summer than in the winter half of the year, and, assuming 
that the breadth of a barometric wave is some function of the 
time taken to complete an oscillation, it follows that at 
Lisbon the areas of disturbance or breadths of the barometric 
waves are greatest in the winter and least in the summer 
months, whilst in England, on the contrary, they are greatest 
in the summer and least in the winter months- It seems 
probable, therefore, that at some intermediate latitude they 
are the same in both halves of the year, and this latitude may 
possibly be the nodal line separating a zone of high barometer 
from one of low barometer. 
Comparing the six years of greatest amount of oscillation 
with the six years of least amount, it is shown that in years 
when the total amount of oscillation is above the average, the 
distribution of the increase in the different months is by no 
means in the proportion of the numbers representing the mean 
daily range of oscillation^ The increase is' much greater in 
the winter than in the summer months, but in April and 
September there is a diminution instead of an increase in the 
