ON THE KEW OBSERVATORY. 349 



7th. At the end of a day (or any given period) K is stopped, the nut i^ is 

 relaxed, and H is drawn back (by pulling i^) to its original place in F. — 

 8th. H, together with h^, are carried into a dark room (or closet), where Y 

 is withdrawn from H and placed in the (warmed) mercury-box. — 9tli. When 

 taken out of the mercury-box, Y exhibits a figure as (e. g,) y^ y'^, fig. 4, 

 which indicates that portion of Y which had been exposed to light (in F), 

 the line y^ being the curve of barometric variation, and the line y'^ its 

 abscissa. — 10th. After the usual washing in the hyposulphite of soda-solution, 

 Y is fitted to the ordinate and tracing-boards (fig. 2, Plate XXI., Report for 

 1849), and the tabulation and tracing processes are proceeded with. 



The compensating apparatus will be readily understood by reference to 

 figs. 3, 4 and 5, Plate XXI., &c. 



6^ is the cistern ; the glass cover of which is accurately fitted on it and 

 clamped, by means of two triangular plates (vide fig. 4) and three long screws. 

 This cover has a neck through which the upper part of B has been passed 

 from below, and the lower part of B being slightly conical (the base of the 

 cone being below), is ground into the neck, so as to suspend the cistern 6* 

 securely. 



b^, a ring through which the tube was slid before a globular enlargement 

 which prevents it from sliding back again was made. 



b*, a piece attached to b^ by two screws, and provided with a little eye, 

 through which a short untwisted skein of silk passes and sustains the whole 

 barometer. 



b^, a ring partially surrounding the tube {vide fig. 5). 

 b^, &c., three adjusting screws for preventing accidental oscillations of the 

 barometer, but not actually touching the tube. 



b^ b'' (fig. 3) are two very old and straight-grained pieces of deal. 

 68 Js^ brass pillars connecting b'^ b^ by means of screws and washers, 

 i" 6^, brass plates screwed upon 6' 6^. 



i" and 6'^ rods of hard zinc, upon the ends of which are soldered brass 

 caps. 



The upper cap of 6" is attached to b^ at (by hypothesis) an invariable 

 point, and its lower cap to a joint at one end of 



i'*, which is a lever whose fulcrum is at about the distance of one-third 

 its length from this joint. The lower cap of 6'- is attached to a joint at the 

 other end (of 6'^), distant two-thirds of its length from the fulcrum. The 

 upper cap of 5'^ is attached to a joint in 



5'*, which is a piece capable of being slid in a mortice by the action of a 

 milled-headed screw in 



6'*, which is a lever whose fulcrum is at the distance of about one-third 

 its length from the joint in 6'*. The left end of 6'* is provided with a piece 

 Curved to radius of its distance from the fulcrum, which distance is equal to 

 two-thirds the length of 6'^, and this arc receives the silken skein which sus- 

 tains B. The fulcrum of 6'* is a hard steel knife-edge working in a hard 

 steel ring. 



b^^ is an index, of thin brass, attached to 6'^ and pointing a scale en- 

 graved on 6^. 



])\i 517 are screws which screw through pieces attached to b'' W, and bear 

 upon plates with sockets adjustable on P P. They are used for small ad- 

 justments for height, perpendicularity, &c. of the whole frame (composed of 

 V, ¥, &c.). 



6'8 {vide fig. 2), a piece projecting from the legs to prevent oscillations of 

 the said frame, but not fastened to it. 



