Three previously unpublished methods for empirically determining deep-water swell direction were examined in this work:(1) triangulation using two widely separated wave sensors, (2) intersection using weather maps and a single wave sensor, and (3)
I. This paper, which is a continuation of a series,' gives the results of a determination of the lunar atmospheric tide a t Buenos Ayres, based on hourly observations* of the barometric pressure made throughout the period of twenty years 1891-1910 (with some few gaps).The observations are derived from a Sprung-Fuess barograph, standardised by a mercury barometer made by Negretti and Zambra. The observatory has a t various times been located at different sites, in and near the city, within a few kilometres of each other, and at a similar small elevation (about 2 5 metres) above sea level. The geographical co-ordinates of the station are approximately 34O.6 S., 58O.4 I?. The barometric pressure, corrected to o°C., is given in the published tables t o 0 1 mm.2. The mean solar diurnal \.ariation for each month was subtracted from the data for that month, and the hourly differences were increased by 6 rnm. to render them-in most cases-positive. They were re-tabulated, t o the nearest whole mm., in lunar daily series of 26, the first entry on any row referring to the solar hour nearest to the time of lunar transit on the day concerned. Sums by columns then gave the combined lunar hourly inequalities of the barometric pressure for the days involved in the summation; in general the 26th term in such a n inequality did not equal the first, indicating the presence of a non-cyclic variation. This was partly due to the seasonal variation of barometric pressure at Buenos Ayres, where in July and August the pressure is, on the average, 4 or 5 mm. higher than in December and January; but the major part of the non-cyclic variation was due to the fact that the daily rows in which the range of the hourly differences exceeded 8 mm. were excluded from the summations ; these barometrically " disturbed " days numbered 15 to 20 per year; they were, on the average, days of rising pressure, and their omission caused a corresponding non-cyclic decrease in the lunar hourly inequalities. T h e non-cyclic variation, which was assumed uniform throughout the day, was removed from each inequality, which was thus reduced to a sequence of 25 terms, omitting the 26th term, now the same as the first.3. Lunar hourly inequalities were formed for the whole set of data, and for numerous subdivisions of it, for the decades 18g1-rgoo and I~O I -I~I O separately, for the three seasons defined as follows :-
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