The observations made by the author were of the three usual elements —the Dip, Declination, and Intensity of the Horizontal Component of the Force. They were taken with instruments supplied to the Colaba Observatory in the year 1867 through the Kew Committee of the British Association, after having been tested at the Kew Observatory. The dip-circle was made by Barrow of London, and is furnished with two needles; the other instrument, the unifilar magnetometer, which serves both for observations of declination and horizontal force, was made by Elliott Brothers of London. The results of the observations for dip only have as yet been received from the author. A complete observation consists of thirty-two readings, each end of the needle being read twice in each different position of the needle and circle; and the mean of the thirty-two is taken as the result of the observation. The observations were 178 in number, commencing on the 29th of April 1867, and extending to the 29th of December 1868. They were generally taken, with the two needles alternately, on particular days of the week. Up to August 17, 1867, the observations commenced with either end (A or B) of the needle dipping, and without remagnetizing the needle;
i. e
. the magnetization for the latter half of one observation was made to serve for the first half of the next observation with the same needle, the two needles having been kept during the interval with contrary poles adjacent in a zinc box; but after August 17, 1867, the needle was always remagnetized, so as to make the end A dip during the first half of the observation. The effect of this change of practice was to produce a marked increase in the accordance of successive observations. Tables are given containing every complete observation made up to the end of 1868, and showing, as well as the mean dip, the partial results in each position of the circle, and with each end of the needle dipping, and also the mean weekly and mean monthly values. The mean dip obtained for the months April to December 1867 was 19° 2'.00, and for the year 1868 was 19° 3'.87. The period embraced by the observations is too limited to allow of an exact determination of the rate of secular change; nevertheless the observations show distinctly that the dip is increasing. The author takes + l'.3 as the rate of annual change.
Estimation of the mean normal measure from flat sections
