120 British Association, 



the materials. Professor Lloyd mentioned, that in the course of the 

 arrangements now making for the erection of a Magnetical Observa- 

 tory at Greenwich, Mr. Airy had rejected bricks in the construction 

 of the building, finding that they were in all cases magnetic, and 

 sometimes even polar. Mr. Lloyd has since confirmed this obser- 

 vation, by the examination of specimens of bricks from various lo- 

 calities ; and though there appeared to be great diversity in the 

 amount of their action on the needle, he met with none entirely 

 free from such influence. 



The building consists of one principal room, and two smaller 

 rooms, — one of which serves as a vestibule. The principal room is 

 thirty-six feet in length by sixteen in breadth, and has projections 

 in its longer sides, which increase the breadth of the central part to 

 twenty feet. This room will contain four principal instruments, 

 suitably supported on stone pillars : viz. a transit instrument, a theo- 

 dolite, a variation instrument, and a dipping circle. The transit in- 

 strument (four feet in focal length,) will be stationed close to the 

 southern window of the room. In this position it will serve for the 

 determination of the time; and a small trap-door in the ceiling will 

 enable the observer to adjust it to the meridian. The theodolite 

 will be situated towards the other end of the room, and its centre 

 will be on the meridian line of the transit. The limb of the theodo- 

 lite is twelve inches in diameter, and is read off by three verniers to 

 ten seconds. Its telescope has a focal length of twenty inches, and 

 is furnished with a micrometer reading to a single second, for the 

 purpose of observing the diurnal variation. 



The variation instrument will be placed in the magnetic meridian, 

 with respect to the theodolite, the distance between these instru- 

 ments being about seven feet. The needle is a rectangular bar, 

 twelve inches long, suspended by parallel silk fibres, and inclosed in 

 a box to protect it from the agitation of the air. The magnetic bar 

 is furnished with an achromatic lens at one end, and a cross of wires 

 at the other, after the principle of the collimator. This will be ob- 

 served with the telescope of the theodolite, in the usual manner j 

 and the deviation of the line of collimation of the collimator from 

 the magnetic axis will be ascertained by reversal. The direction of 

 the magnetic meridian being thus found, that of the true meridian 

 will be given by the transit. It is only necessary to turn over the 

 transit telescope, and, using it also as a coUimator, to make a simi- 

 lar reading of its central wire, by the telescope of the theodolite. 

 The angle read off on the limb of the theodolite is obviously the 

 supplement of the variation. This use of the transit has been sug- 

 gested by Dr. Robinson j and it is anticipated that much advantage 

 will result from the circumstance, that the two extremities of the 

 arc are observed by precisely the same instrumental means. With 

 this apparatus it is intended to make observations of the absolute va- 

 riation twice each day, as is done in the observatory of Professor 

 Gauss, at Gottingen, — the course of the diurnal variation, and the 

 hours of maxima and minima, having been ascertained by a series 

 of preliminary observations with the same instrument. 



