222 MESSRS. K. THKELFALL AND J. A. POLLOCK 



It is necessary to observe either upon a stone, cement, earthen or asphalte floor. 

 Boards, however strong, do not form a sufficiently inelastic support. 



INSTRUMENTAL DETAILS. 



The drawings on Plate 1 3 are to scale, and ought to be self explanatory. Notes, 

 however, have been added on the page facing the drawings in order to make it 

 easier to follow the latter. It will be seen that the general principle of construction 

 is as follows : All the essential parts are rigidly held together by a system of bars 

 forming a complete mechanism. This is then thrust into a tube of copper, which it 

 fits precisely, and the tube of copper is further surrounded by a packing of paper to 

 insure some degree of thermal insulation. The paper in its turn is surrounded by an 

 outer tube of brass, and this is held down to the base by brass clips. The copper 

 tube is closed at one end by the brass cover carrying the vernier arm and mercury 

 stuffing box, which constitute the circle end of the machine. At the other end there 

 is a smaller brass cover, through which projects the end of the arrester shaft and the 

 end of the thermometer ; or, rather, the thick terminals and the end of the ebonite 

 shank on which the glass tube is mounted. The aneroid tube also passes to the 

 interior of the apparatus through this end. 



The copper tube has two openings at opposite ends of a horizontal diameter, and 

 into these openings are fastened, at one side a window, at the other the microscope 

 focussed upon the end of the lever. 



The under frame is provided with levelling screws so spaced as to fit the grooves of 

 the top of a Kew magnetometer tripod. The instrument having been reconstructed 

 several times does not now occupy the best position on the under frame, and this has 

 necessitated the addition of a heavy lead counterpoise. Without this counterpoise 

 the weight carried by the back levelling screw is too great to permit of the screw 

 being turned with sufficient ease for exact levelling. The counterpoise is not shown 

 in the drawing, but appears in the photograph (see Plate 14). 



Attachment of the inner mechanism to the copper tube. The bars connecting the 

 bearing (which supports the " spring" end of the thread) to the bearing which carries 

 the vernier arm are of gun-metal, and the bearing which carries the spring fits with 

 considerable accuracy into the copper tube, carefully bored for the purpose. During 

 our earlier observations the spring bearing was quite free to slide up or down the 

 copper tube according as the temperature rose or fell. After one of our journeys, 

 however, we thought that there was some evidence of the spring end of the thread 

 having moved, and it appeared possible to connect this motion with the freedom of 

 the spring bearing. Accordingly at present the bearing is wedged tight to the copper 

 tube. 



Stuffing Boxes. 



The idea was to render the brass work incapable of being amalgamated by plating 



