rometrical registers of Port Macquarie, Port Jackson, and Port Philip, began only so late as 1840, and that the records of that year being incomplete, and the barometrical observations of the following year being interrupted at Port Macquarie by an accident which happened to the instrument, the only registers that furnish any barometrical data are those of Port Jackson and Port Philip, for the two years ending with 1842. In Van Diemen's Land, the stations of Port Arthur, Circular Head, and Woolnorth each furnished their quota of barometrical observations, which exhibit, throughout five years, ending with 1842, a most praiseworthy perseverance, accuracy, and good faith, on the part of the observers. To the above registers may be added my own, kept for four years, ending with 1842, and which, beside the daily observations of the barometer, furnishes some horary ones, undertaken with the view of ascer taining the rate of horary oscillation of the atmospheric tides. The period which these registers embrace is trifling, compared to that which is required for obtaining a sufficiency of material for meteorological influences: the intervals also between the observations are too great, separating them too widely, and consequently limiting the range of their application. Still, as they form the first record of a series of barometrical observations on the Australian continent, they deserve to be examined, if but to show the great importance which an accumulation of similar observations would have on future speculations. As a preparatory step to their appreciation, the numerical elements which they give have been reduced to one common term of comparison, and exposed in tabular forms most appropriate to illustrate the phenomena to which they relate; a labour which can only be appreciated by those meteorologists who have reduced and computed similar observations. Three tables have resulted from the computations: the first includes the means of bihorary observations, the hours marked with an asterisk being interpolated. The second gives the mean height of the barometer, and its mean monthly oscillation, for the two seasons of each year. The third and last brings the means of seasons to the last term of averages. Diurnal Variation of the Barometer. - From the inspection of the first, second, and third column of Table III., it is evident that the average of five, ten, or fifteen days of consecutive observations (day and night) embraces too limited a series for the discovery of a law in connection with this variation. The average of twenty days in July rendered a regular oscillation of the barometer in some degree apparent, and has led to the belief that the hours of such oscillation are only perceptible in an average of a series of observations extending over more than twenty days. Thus the fifth column embraces twenty-five days, and shows pretty clearly that the barometrical oscillation in August assumed a maximum at noon, a minimum at three p. m., a maximum again at seven, and again a minimum at eleven. After midnight, at one o'clock, the barometer attained a maximum; at three it came to a minimum; at five, to a maximum ; at seven, to a minimum; and at eleven, to a maximum again. Consequently, in twenty-four hours, the barometrical oscillation exhibited nine phases, four of which were minimum, and five maximum. Computing now, from these oscillations, the mean of the diurnal variation, after Humboldt's method, that is, taking the difference between the two ex tremes of morning and evening, and that variation would be only = 0·085. Interesting as the subject is, its investigation is nevertheless inaccessible to the best efforts of a single observer, upon whom the duty of other observations and labours may devolve. We must therefore content ourselves for the present with conjectures only upon the developement of the laws of diurnal variation, and wait for the final solution of the question the results of the labours that are going on in the Hobart Town observatory, where, as we have already said, ardent and indefatigable observers are recording, night and day, all the phenomena connected with terrestrial physics. CIRCULAR HEAD. PORT ARTHUR. TABLE IV. OF THE MEAN ATMOSPHERICAL PRESSURES AND MEAN BAROMETRICAL OSCILLATION FOR THE TWO SEASONS OF DATE. WOOLNORTH, Pressure. Oscillation. Pressure. Oscillation. Pressure. Oscillation. Pressure. Oscillation. Pressure. Oscillation. PORT JACKSON. PORT PHILIP. 29.951 00.830 29.735 00.971 29.728 00.800 |