416 



On the Tides of Bombay and Kurrachee. 



[May 28, 



quality in time, which were necessary to the previous process, being now 

 altogether disregarded. 



The diurnal inequalities in height were obtained by measuring the widths 

 of the brown spaces where they were crossed by the vertical lines repre- 

 senting noon on successive days. The two daily values thus obtained are 

 respectively the sine and cosine of an angle which represents the difference 

 in time between semidiurnal and diurnal tide. Dividing the low-water by 

 the high-water value gives the cotangent of that angle, and thence the 

 angle itself. Thus the time of actual diurnal tide (first in relation to the 

 time of semidiurnal low water, and then in relation to solar time) was 

 obtained. 



The actual range of diurnal tide was obtained by adding together the 

 squares of the high-water and low-water values (sine and cosine), and 

 taking the square root of the sum. 



With these two series of results as ordinates, curves were drawn repre- 

 senting times and ranges of actual diurnal tide, which were thus presented 

 in a convenient form for comparison with the diurnal tide which had been 

 previously calculated. 



The comparison confirmed the previous conclusion that the tide based 

 on the simple declination theory was insufficient, and the empirical correc- 

 tion which had been adopted seemed to provide an approximation to the 

 required addition to it, both in time and height. But it appeared that a 

 better coincidence in time would have been obtained by assuming the diur- 

 nal tide at Kurrachee to be forty minutes earlier. This supposition was 

 tested by treating the observations of 18G5 in a similar manner, and also 

 by recalculating a portion of the tides of 1867 with the earlier diurnal 

 tide. In both cases the supposition was confirmed, a better agreement 

 being obtained. ■ 



On treating the Bombay observations in the same manner, a fair general 

 coincidence with the calculated diurnal tides was found to exist ; but it 

 was further found, on comparing together the Kurrachee and Bombay- 

 curves of actual diurnal tide (thus for the first time recorded for the same 

 period), that the times were nearly identical at the two ports, and the 

 range at Bombay about one-tenth greater than that at Kurrachee. 



The tables for the four months over which the Bombay observations ex- 

 tend were recalculated with the diurnal tides which had been calculated 

 for Kurrachee (but made forty minutes earlier, and increased in range by 

 one-tenth), and the result was quite as good as that shown by the original 

 tables. This fact would seem to point to the possibility that the diurnal 

 tide is a vertical undulation, acting simultaneously, or nearly so, over a 

 large area. 



