HISTORY OF PHYSICAL ASTRONOMY.

163

be some small island in the middle of the ocean, because the barometer■would in that case be least liable to be affected by the fluctuations arisingfrom the irregularities of the earth’s surface. With the view of throwingsome light on this delicate subject Captain Lefroy, of the Koyal Artillery,undertook a series of barometrical observations at the island of St. Helena .These observations were conducted solely with reference to the place ofthe moon, as the effect of the sun’s influence was naturally expected to beinsensible. They extended from August, 1840, to December, 1841, andtherefore comprised a period of seventeen months. The mean result ofthese observations clearly indicated the existence of a lunar atmospherictide. It gave 38.2714 inches for the height of the barometer when themoon was on the meridian, and 28.2676 when she was on the horizon. Thedifference was therefore .0039 inches*. The observations were subse-quently resumed by Captain Sm^tlie in October, 1842, and were continuedtill September, 1843. The average of all the results obtained during thisperiod gave .00265 inches for the excess of the altitude of the barometer,when the moon was on the meridian, over the altitude when she rvas sixhours distant from it. The observations for the following two years werecompared together by Lieutenant Colonel Sabine at Woolwich, and theresults he derived from them presented a satisfactory accordance withthose previously obtained by Lefroy and Smythe. The average excess ofbarometrical pressure during this period amounted to .00305 inches, or inround numbers to .004 inches. It is manifest that the effect of the moon’saction ought to be greatest when she is in perigee, and least when she isin apogee. This is unequivocally indicated by the observations, as appearsfrom the following results of the mean barometrical excess, obtained bya comparison of observations made when the moon was on the medidian,and when she was six hours distant from it:—

Mean excess of pressure derived—

from 13 epochs of perigee, between 1Oct. 1843 and Sept. 1844. j

from 13 epochs of perigee, between 1Oct. 1844 and Sept. 1846. j

from 13 epochs of apogee, between )

Oct. 1843 and Sept. 1844. j

from 14 epochs of apogee, between \

Oct. 1844 and Sept. 1845. j

These quantities are small, but still they are sufficiently sensible toestablish beyond doubt the existence of oscillations in the atmosphere,similar to those which affect the waters of the ocean. This is not theleast interesting of the many facts in physical science which vvould havefor ever escaped detection, if their existence had not been suggested bythe theory of gravitation.

* Phil. Trans., 1847.

.00407 inches..00394 „

.00341 „

.00347 „