V. On the Lunar Atmospheric Tide at St. Helena. 
By Lieut. -Colonel Edward Sabine, R.A., For. Sec. R.S. 
Received December 16, 1846, — Read January 28, 1847. 
THE attractions of the sun and moon occasion tides in the atmosphere similar to 
those of the ocean. The effects are, however, so inconsiderable in comparison with 
the disturbances produced in the equilibrium of the atmosphere by other causes, that 
hitherto observation has not succeeded in affording any clear or decisive evidence 
of them. The phenomenon however has a very considerable philosophical interest 
as a farther exemplification of the universality of the principle of gravitation. 
As the ebb and flow of the atmospheric tide must be greatest in the vicinity of the 
equator, and as moreover the ordinary equilibrium is less disturbed by irregular 
causes in that region of the globe than in the temperate zones, it was reasonable to 
expect that the existence of a tide in the atmosphere, depending on the position of 
the sun or moon relatively to the meridian of the place, would be shown, if at all, by 
observations made within the tropics ; and that the most favourable situation for such 
observations might be that of a small island surrounded by a considerable extent of 
ocean, and therefore comparatively free from the atmospheric disturbances occasioned 
by variations of the terrestrial surface ; and where also the tides of the ocean should 
be small. 
The importance of St. Helena as a station of observation for this purpose early 
engaged the attention of the highly intelligent director of its magnetical and meteo- 
rological observatory, Captain Lefroy of the Royal Artillery, as may be seen from 
the following extract of a report received from that officer, dated June 1st, 1842. 
“ I believe that no observations have as yet decided the question, whether any 
effect upon the mean barometrical pressure is produced by the moon’s daily passage 
of the meridian. The existence of an atmospheric tide of this nature appeared how- 
ever so interesting a subject of inquiry, and its detection so probable, owing to the 
extreme uniformity of the daily oscillation, that the observations of seventeen months, 
viz. from August 1840 to December 1841 inclusive, have been arranged with that 
view. The mode adopted was this : — the corrected height of the barometer (i. e. the 
reading reduced to32°FAHR.) at the hour of observation nearest the moon’s meridian 
passage for every day has been entered in a central column ; and in parallel columns 
headed — 2 h , — 4 h , &c. and +2 h , +4 h , &c. have been entered the observations taken 
respectively at 2 h , 4 h , &c. before and after the central observation. A mean has been 
taken for the observations included in each lunar month. It appears from the 
