Declinometer. xi 



diurnal series of observations. Sir Thomas, with his usual anxiety to render the 

 Observatory in every respect useful to science, at once complied with my recom- 

 mendation ; and, in the beginning of the year 1844, hourly observations were com- 

 menced, which were continued till the end of the year 1845. The whole stafF of 

 observers, after the commencement of 1844, therefore, consisted of Mr John Welsh, 

 Mr Alexander Hogg, and myself; Mr Dods assisting in the term-day observa- 

 tions, and on a few occasions during disturbances. 



Declinometer. 

 6. The declination magnetometer was obtained from Grubb of Dublm. 



The 



magnet a is 15 inches long, i inch broad, and l inch thick ; it fits into a stirrup b, 



whose two eyes receive an axle to which the suspension thread is attached ; near 

 the north extremity it carries a scale divided on glass, c ; near the other, at a dis- 

 tance from the scale of about 12 inches, the focal length, it carries a lens of 1| inch 

 diameter, d. A marble slab m, cemented to the top of the stone pillar p, carries two 

 copper tubes//, 35 inches long, which are connected at the top by a mahogany 

 tie g, bearing the torsion-circle and suspension apparatus t, and, about 7 inches 



