132 Dr. Watson on the Determination of the 



the position o£ the bar B, with reference to the cradle, can be 

 adjusted by means o£ the screw J, and yet, when the cradle 

 is released, the bar may swing clear of the point of this screw. 

 Two small weights D serve to adjust the balance and period 

 of the cradle. 



The quartz fibre is soldered to a clip E which hooks on 

 to the cradle and another clip which hooks on to the end 

 of a rod carried by the torsion-head L. An arm, M, 

 attached to the torsion -head can be moved between two 

 adjustable stops N. This arm serves to give the torsion-head 

 a small to-and-fro motion of adjustable and fixed amount, 

 and thus allows of the bar being set swinging; 



The bar, &c.,is enclosed in a wooden box the sides of which 

 are removable. A window P allows of the mirror being seen; 

 while a hole Q, which can be closed by a cork, serves for the 

 removal of the bar from the cradle. The torsion-head is 

 supported by a brass tube 30 cms. long attached to the top of 

 the box. This brass tube is wound round wdth a layer of felt 

 to reduce the temperature changes . 



The method adopted for adjusting the instrument is as 

 follows. The base of the instrument having been levelled, 

 the cradle is supported on the table with the bar in position, 

 and a striding level is placed on the bar. By adjusting the 

 screws Gr and H the bar is brought into a horizontal position, 

 and a telescope and scale are set np at a distance of about 

 70 cms. from the instrument, and so adjusted that the horizontal 

 cross wire coincides with the ends of the division-lines of the 

 scale as seen reflected in the mirror C. The bar is then 

 removed and the cradle released. The weights J) are adjusted 

 till the horizontal wire of the telescope again coincides with 

 the images of the ends of the division-lines of the scale. When 

 this adjustment is complete the line joining the Y ; s of the cradle 

 will be horizontal. The bar having been placed in position, 

 the screw J is adjusted till on releasing the cradle the hori- 

 zontal wire of the telescope again coincides with the ends of 

 the division-lines of the scale, and therefore the axis of the 

 bar is horizontal. 



The vertical cross wire having been brought into coin- 

 cidence with some well-marked division of the scale, the arm 

 M is moved from its position against one of the stops X up to 

 the other stop and back again, thus starting the bar swinging. 

 The two thermometers T are read, and the period is determined 

 by means of a chronometer in the usual manner. The fol- 

 lowing is an example of such a determination of period, 

 though in some of the observations, particularly those in 

 which the air effect was being investigated, the vibrations 

 were allowed to continue for over 30 minutes. 



