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I. " On the Lunar Semidiurnal Variation of the Barometer." 

 By JOHN ALLAN BROUN, Esq., F.E.S., Director of the Tre- 

 vandrum Observatory. Received May 30, 1861. 



The results obtained hitherto for the lunar diurnal variation of 

 atmospheric pressure have been received with some doubt ; the range 

 of the variation has been found to be small, and the proximity to the 

 sea of the stations for which the discussions have been made, has 

 given some value to the idea that the variation is due simply to the 

 tidal rise and fall of the sea, the base of the atmosphere. It has 

 even been suggested that the barometer should be incapable of show- 

 ing an atmospheric tide, because the gravity of the mercury is dimi- 

 nished by the lunar attraction, as well as that of the atmosphere ; 

 this suggestion, however, omits all consideration of the integrating 

 means by which a tide is formed. 



I have considered the question at two stations in the following 

 manner. Simultaneous observations of two standard barometers of 

 the same construction, with boiled tubes 0'65 inch internal diameter, 

 were made hourly during fifteen months (April 1857 to June 1858) 

 at two observatories ; one at Trevandrum 200 feet above the sea- 

 level and three miles distant from the sea, the other on the summit 

 of the Agustier peak of the South Indian Ghats, 6200 feet above the 

 sea, about 22 miles from Trevandrum, 25 miles from the sea on the 

 west, and 40 to 60 miles from the sea on the south and east. The 

 means of all the observations give the following results. 



At Trevandrum. The lunar diurnal variation of atmospheric 

 pressure has two nearly equal maxima, occurring almost exactly at 

 the moon's passages of the upper and lower meridian, the minima 

 occurring six hours before and after these epochs. 



At Agustier (6200 feet}. The law is nearly the same as to epochs, 

 the maximum for the inferior passage occurring, however, about one 

 hour later, and each minimum occurring about one hour nearer the 

 upper passage. 



The chief difference in the two results is that between the relative 

 values of the maxima and minima. The oscillation occurring while 

 the moon is below the horizon, or between the meridians of 6 h and 

 of 18 h , has the same amount at Trevandrum (height 200 feet) and 

 at Agustier (height 6200 feet) ; but the oscillation occurring while 



