226 Instructions for the Scientific Expedition 



In addition to the parts abovementioned, the instrument is pro- 

 vided with a second magnet, of the same dimensions as the first, to 

 be used in measurements of absolute intensity ; a thermometer, the 

 bulb of which enters the box, in order to determine the interior tem- 

 perature ; and a copper ring, for the purpose of checking the vibra- 

 tions. ' 



Adjustment. — The instrument having been placed on its support, 

 the base is to be levelled, and the whole then fixed in its place. The 

 levelling of the base may conveniently be performed by the aid of a 

 plumb-line hanging in the place of the suspension thread ; but no 

 great precision is required in this operation, the chief object of which 

 is that the suspension thread may occupy the middle of the tube, and 

 that the magnet may be central with regard to its support. The 

 suspension thread is then to be formed, and attached at one extremity 

 to the roller of the suspension apparatus, and at the other to the 

 small cylinder which is to bear the stirrup and magnet. Sixteen 

 fibres* of untwisted silk are sufficient to bear double the load with- 

 out breaking, and will be found to form in other respects a conve- 

 nient suspension. 



These preparations being made, the adjustments are the following: 



1 . The sliders being placed on the magnet, the scale is to be ad- 

 justed to the focus of the lens, and in such a manner that the centre 

 of gravity of the sliders may be near the middle of the bar. The 

 adjustment to focus has been already made by the artist, and the 

 corresponding distances of the sliders measured ; they will be found 

 in Table I. 



2. The magnet is to be connected with the suspension thread by 

 means of the stirrup, and to be moved in the stirrup until it assumes 

 the horizontal position. This adjustment may be conveniently ef- 

 fected by means of the image of the magnet, reflected from the sur- 

 face of water or mercury, the object and its reflected image being 

 parallel when the former is horizontal. The stirrup is then fastened 

 by its screws, and the magnet wound up to the desired height. As 

 the thread stretches considerably at first, allowance should be made 

 for this in the height. 



3.t The magnet is then removed, and the unmagnetic bar (having 

 its collimator similarly adjusted) is to be attached, without its small 

 magnet, and allowed to swing for several hours. The bar having 

 come to rest, or nearly so, its deviation from the magnetic meridian 

 is to be estimated, and the moveable arm of the torsion circle turned 

 through the same angle in an opposite direction. The plane of 

 detorsion then coincides, approximately, with the magnetic meri- 

 dian. 



4. The magnet is then to be substituted for the unmagnetic bar, 

 and the telescope being directed towards the collimator, the point 

 of the scale coinciding with the vertical wire is to be noted when 



* Not the individual fibre of the silk-worm, but the compound fibre in the state 

 in which it is prepared for spinning. 



t It is obvious that this step of the adjustment may precede the 1st and 2nd, 

 where a saving of time is important. 



