We have data of magnetic observations at Colaba, Bombay, from 1846 to 1905, and at Alibag, 18 miles to the south, from 1904 onward, with two years of comparative observations during 1904 and 1905. Moos had studied in 1910 the secular variations of the magnetic field with data up to 1905. In view of the changes in the magnetic field and the very large amount of data collected since, a further study was undertaken.
The value of H has been increasing continuously since 1916, and it is likely that the increase will continue at least for some more years. The westward drift of declination commenced, from 1879 and is still continuing. The rate of drift, however, has decreased during the last five years, and within the next few years it may begin to return eastward or move westward at an increased rate. The vertical force had been continuously increasing from 1853 and it reached its maximum value in 1937, after which there was a fall for some years, but the falling tendency appears to have been arrested and the vertical force may again begin to increase. During the first six solar cycles, the average values of H, ΔH, and Δ′H are greater during sunspot minimum years than during sunspot maximum years, but this does not hold during the last three cycles. No periodicity of about 11 years was apparent in the curves representing the differences between the observed and calculated values of H, D, and V or in the curves representing ΔH, Δ′H, ΔD, Δ′D, ΔV, and Δ′V, and no parallelism was noticeable between these curves and the sunspot curve.