388 Mr. V. N. Nene. On a Method of Tracing 



pressure for the 24 years. If we add the constant 29'808, the annual 

 mean of 26 years, 1847 to 1872, to the 12 numbers in our Table II, 

 then these numbers will be found almost identical with the numbers 

 at the foot of Table I, given in the Meteorology of the Bombay 

 Presidency. The 12 numbers at the foot of Table II were then 

 subtracted in order from each of the rows of 12 numbers commencing 

 from January, and ending with December for each of the years 1848 

 to 1877 of the same table, and the remainders entered in Table IV. 

 Again, with the numbers in Table IV, we have performed three 

 operations, taking 3 as our working value of m to get the third 

 difference and sum series, which we have given in Table V, then the 

 numbers in Table V were subtracted from the corresponding numbers 

 in Table IV, and entered in Table VI. 



We need not say that the sum of the numbers in Tables III, V, 

 and VI, from July, 1848, to June, 1876, and successive repetitions of 

 the corresponding numbers of annual variation at the foot of Table II, 

 taken in order, will be equal to the corresponding numbers in Table I. 

 The numbers in Tables III and VI are curved in figs. 2 and 3 respec- 

 tively. We presume that the curve of fig. 2 is composed of periods of 

 more than 12 months, and that of fig. 3 is of less than 12 months and 

 greater than three months. By looking at the oscillations of the 

 curve of fig. 2 we suspect that amongst the periods there are periods 

 of 11 or 12 years, 4J years, and 2 years. By looking at the oscillations 

 of the curve of fig. 3, we see that amongst the periods there is a great 

 predominance of a 9^-rnonthly period. This we have ascertained in 

 the following manner : — We have first of all marked by a cross 

 (thus X ) all the minima that have their time-interval from one 

 minimum to next minimum 8 to 11 months, or any integral multiple 

 of 8 to 11 months, and then filled up the rest 8 to 11-monthly minima 

 that were less marked by a mark (thus 0) . It is remarkable that the 

 oscillations are more prominent and of greater amplitude from the 

 year 1849 to 1862 than from the year 1863 to 1876. If we count the 

 marks of minima commencing from July, 1848, and ending with 

 November, 1876, they will be found to be 37; thus there are 36 

 periods in 340 months, i.e., 9-J months, as an average duration for the 

 period. 



42. Again, we find that there are intermediate minima which are 

 of less amplitude than already stated, and the duration of which is 

 also from 8 to 11 months. These also we have marked by marks 

 (* and 0), as in the preceding example. In the present case both these 



series from 1847 to 1872, and have made all the operations we are going to describe. 

 But afterwards it was thought proper to add to the above result the six years 1873 

 to 1878, which were available. But as the work was far advanced it was not 

 worth while to go into the trouble of doing the work over again for the sake of 

 including those six years. 



