Mr. J. A. Broun on the Lunar Barometric Variation. 67 



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 

 the moon is above the horizon, has twice the amount at the lower 

 station which it has at the upper station. 



This curious result, which it is difficult to explain by any theory 

 depending simply on the attraction of gravitation, whether acting on 

 the sea or the atmosphere, agrees remarkably with the conclusions 

 for the solar diurnal oscillation obtained from the discussion of 

 hourly observations made simultaneously during a month at five 

 different stations, four of these being on the Agustier mountains, 

 rising gradually from 1200 to 6200 feet, the fifth station being at 

 Trevandrum. From these observations (an abstract of which will 

 be found noticed in the Report of the British Association for 1859, 

 Trans, of Sect., p. 46), it appeared that the solar semidiurnal oscilla- 

 tion between 9 p.m. and 9 a.m. was nearly the same at all the 

 stations; while the day oscillation, 9 a.m. to 9 p.m., diminished 

 with the height, being at 6200 feet little more than half the amount 

 of the oscillation at 200 feet. 



These facts might be put into the following general form :— When 

 the heavenly body (sun or moon) is below the horizon, the semidiurnal 

 oscillation of the barometer within the tropics has the same value at 

 all heights up to 6200 feet ; but when the body is above the horizon, 

 the oscillation at 6200 feet has only half the amount of the oscillation 

 at the level of the sea. 



" On the Law of Disturbance and the Range of the Diurnal Varia- 

 tion of Magnetic Declination near the Magnetic Equator, with refer- 

 ence to the Moon's Hour-angle." By John Allan Broun, Esq., F.R.S. 



The discovery by Dr. Lamont of a "decennial" period in the 

 range of the solar diurnal variation of magnetic declination, naturally 

 leads to the question whether a similar law may not exist for the 

 lunar diurnal variation ; the question is also of importance in con- 

 nexion with the theory of the cause of these variations. The follow- 

 ing results for the range of the lunar diurnal variation were obtained 

 from the discussion whose conclusions were given in the Proceedings 

 of the Royal Society, vol. x. p. 4/5. 



Range of Diurnal Variation at Trevandrum, 1854-59. 





Lunar. 



Solar. 







From variations 



From variations 



Ratio. 





for groups of 



for single 



Solar 



Year. 



3 months. 



months. 



Lunar. 



1854 .. 



0-52 ... 



2-24 .... 



4-3 



1855 .. 



0-54 ... 



2-05 .... 



3-6 



1856 . . 



0-41 ... 



201 .... 



4-9 



1857 .. 



0-53 ... 



2-15 .... 



4-0 



1858 . . 



0*58 ... 



2-41 .... 



4-2 



1859 .. 



0-66 ... 



2-64 .... 



40 



F2 



