68 



Monday^ bth January 1846. 



Sir T. MAKDOUGALL BRISBANE, Bart., President, in 

 the Chair. 



The following Communications were read: — 



1. On a new variety of Gamboge from the Wynaad. By Dr 



Christison. 



2. Results of the Makerstoun Observations, No. 1. On the 



Relation of the Variations of the Earth's Magnetism to 

 the Solar and Lunar Periods. By J. A. Broun, Esq. 

 Communicated by Sir T. M. Brisbane, Bart. 



The observations from which the following results are deduced, 

 were obtained during the years 1844 and 1845, by means oi the 

 bifilar magnetometer, described in the Makerstoun Observations for 

 1841 and 1842 ; they are corrected for temperature by a method 

 previously described.* 



From the Observations for the year 1844, the diurnal variation 

 of the horizontal force is found to consist of two maxima and two 

 minima; the minimum occurs at 10*^ 20™ a.m. (Makerstoun mean 

 solar time is used throughout), the maximum at 5*" 30™ p.m, ; a 

 minimum occurs at 2*^ 20™ a.m., and a maximum at fi^ 30™, or 40™ 

 A.M.: two inflexions occur in the mean curve between Z^ and 4'' p.m., 

 and between 8'^ and 9'' p.m. The periods of maxima and minima 

 vary throughout the year, and the morning maximum is greater 

 than the afternoon maximum in winter. 



The diurnal range is greatest in July and least in January ; the 

 mean of the ranges for these two months, the range for the month of 

 March and for the month of October, are each nearly the mean 

 diurnal range for the year. 



The author shewed to the Physical Section of the British Associa- 

 tion in June 1845, that the Makerstoun Observations for 1842 in- 

 dicated a well-marked annual period of horizontal force, consisting of 

 maxima near the solstices, and minima near the equinoxes ; in cor- 

 recting the Toronto Observations for 1842, by the method already 

 noticed, he had arrived at the same result. He has now verified 

 this period from the Makerstoun Observations for 1843, 1844, and 

 1845. 



* Transactions of the Royal Society of Edinhurgli, Vol. XVI. Part. 1. 



