4 British Association for the Advancement of Science. 



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 toward the other end of the room, 

 and its centre will be on the meridian line of the transit. The 

 limb of the theodolite 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 dimmed variation. 



The variation instrument will be placed in the magnetic merid- 

 ian, with respect to the theodolite, the distance between these in- 

 struments 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 observed with the telescope of the theodolite, in the 

 usual manner ; 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 neces- 

 sary to turn over the transit telescope, and, using it also as a 

 collimator, to make a similar 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 suggested by Dr. Robinson ; 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 variation twice each day, as is 

 done in the observatory of Prof. 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. 



A dipping circle constructed by Gambey, will be placed on a 

 pillar at the remote end of the room ; and will be furnished with 

 a needle, whose axis is formed into a knife-edge, for the purpose 

 of observing the diurnal variations of the dip. Gauss's large ap- 

 pai-atus will also be set up in the same room, and will be used 

 occasionally, especially in observations of the absolute intensity, 



