144 



then supposed to be discovered in a bad principle in the mode in 

 which the adjustment for level was contrived. 



The experimentum cruets which the author proposes, is to do away 

 with the adjustable bearings, and have plain simple blocks of brass in 

 their stead, trusting to calculations and not to screws for correcting 

 the instrument. But, as the adoption of this proof will take up a long 

 period, he, in the meanwhile, shews, that this cause which he thinks 

 to have discovered is a vera causa, is sufficient, so far as the quantity 

 is concerned, and is agreeable as to the direction in which it would 

 act : and as a farther test that the disturbing cause is something near 

 the instrument, and not the rocks outside, the readings of the ther- 

 mometer attached to the barometer, and the outer thermometer read 

 off during the mural circle observations were discussed, and gave the 

 following highly confirmatory results : — 



Barometer-thermometer, first half of year: second half: : 1 : 1"220 

 Outer -1:1:114 



4. Results of Makerstoun Observations No. III. On the 

 Solai' and Lunar periods of the Magnetic Declination. 

 By J. A. Broun, Esq. Communicated by Sir T. M. 

 Brisbane, Bart. 



The absolute westerly declination at Makerstoun, for the mean 

 epoch, June 1844 = 25^ 17'- 12. 



The annual motion of the north end of the needle towards the 

 east = 5'*67. 



The annual period of magnetic declination consists of a double 

 oscillation, having nearly the following epochs of maxima and mi- 

 nima.* 

 A max. Jan. 30. The min. Ap. 30. The max. Sep. 10. Amin. Dee.lO. 



The author examines Cassini's observations (1783-7). Although 

 they confirm this law to some extent, it is not conceived that they 

 can be trusted for such a determination. The author also verifies 

 his result by grouping a large mass of modern obsei'vations. The 

 observations at Washington and Toronto, with other facts, prove that 

 the oscillation is inverted, when the secular motion of the needle has 

 an opposite sign, and Colonel Beaufoy's observations (1817-20) seem 

 to prove, that when the secular motion is zero, the annual period is 



* By maximum is always meant an extreme tiestcrh/ position of tlic north end 

 of the needle, and by minimum an extreme easterly (or rather uortherly) posi- 

 tion. 



