59 



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 



