xii Introduction to the Makeestoun Observations, 1843. 



day and extra magnetical and meteorological observations, of observations of mag- 

 netic dip, and of the absolute horizontal magnetic force. 



It will be allowable for me to express here, how much I owe, in the conduct of 

 the Observatory, to the unceasing kindness of its founder and supporter. Sir Thomas 

 Brisbane, as well as to his suggestions, and his uniform attention to every proposal 

 that might in any way tend to the advancement of science. I owe a like acknow- 

 ledgment to Professor J. D. Forbes, and also to Dr H. Lloyd. 



§ 3. Declinometer. 



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

 magnet is 15 inches long, ^ inch broad, and ^ inch thick. It fits into a stirrup, 

 whose two eyes receive an axle attached to the suspension thread. Near the north 

 extremity, it carries a scale divided on glass ; near the other (about 12 inches, the 

 focal length, distant from the scale) a lens of 1^ inch diameter. A marble slab, 

 cemented to the top of the declinometer-pillar, carries two copper tubes, 35 inches 

 long, which are coimected at the top by a mahogany tie, bearing the torsion-circle 

 and the suspension apparatus ; and, about 7 inches from the slab, by another 

 wooden cross-piece, which supports a glass tube enclosing the suspension thread ; 

 the latter cross-piece, together with two glazed lids, fitting on a wooden drum, com- 

 pletes the enclosure of the suspended magnet, and of the copper ring used for check- 

 ing the vibrations of the magnet. There are two glazed apertures in the sides of 

 the box ; one to the north, where a small mirror throws in light upon the glass scale; 

 the other to the south, between the lens and the reading telescope. Previously 

 to September 1843, the glazed lids fitted loosely on the box, and the latter did not 

 rest closely on the marble slab, so that the magnet was probably affected by exter- 

 nal currents of air. In June 1843, a rectangular pasteboard box, open at the ex- 

 tremities, was placed within the cylindrical box and round the magnet, which would 

 have some effect in destroying internal currents : in September 1843, a wooden 

 box was substituted, formed of two pieces fitting into each other in the middle by a 

 groove and tongue, glazed at the extremities, and having only a small aperture for 

 the suspension thread : at the same time, all the joints of the outer box, as well 

 as the lower edge of the inner box, were covered with velvet, and the boxes were 

 pressed firmly against the marble slab by means of leaden weights, which were pre- 

 viously determined to have no effect on the position of the magnet. In order, also, 

 to destroy any effect of radiation in the formation of aerial currents, both boxes were 

 covered with gilt paper, internally and externally. 



7. The pillar of the azimuth circle and transit used for determinations of the 

 absolute declination, is between the pillars of the magnetometer and its reading 

 telescope. The theodolite is by Troughton; the circle is 15 inches in dia- 



