TRANSACTIONS OF THE SECTIONS. 415 



The straight part of the tube, exclusive of the bulb, is 32 

 inches ; inclusive of the bulb, 34 ; the recurved end is bent 

 twice at right angles, so as to project from the tube 3 

 inches, and rise parallel thereto 7-5 inches. The tube is at- 

 tached to a mahogany support, the spheroidal bulb being up- 

 wards ; and the quantity of mercury is so adjusted in the tube 

 that at mean pressures the upper level is nearly coincident 

 with the greatest diameter of the spheroid, and the lower is 

 near the middle of the shorter leg. 



There is a circle of brass, divided into 1000 parts, fixed to 

 the front of a light copper drum or case, having a glass front 

 and back, the centre of which circle is placed just over the 

 orifice of the glass tube : a small frame of brass is fixed to the 

 circle behind, so as to carry a light horizontal axis bearing two 

 small pulleys. The extremities of this axis are turned to ex- 

 tremely fine pivots, and are set in small jewels : the front one 

 projects forward so as to carry a light index of straw, which is 

 sustained on a small brass ring, placed by means of a socket on 

 the extremity of the axis, in the manner of the hand of a watch. 

 The two small pulleys above mentioned carry, by means of 

 fine untwisted silk threads, two small cones of glass or wood, 

 one of which rests on the surface of the mercury in the recurved 

 tube, the other hangs freely on the outside of the tube. These 

 cones are nearly equal in weight, that resting on the mercury 

 being rather the heavier of the two. This slight difference of 

 weight, setting aside the inertia and friction of the axis, is the 

 amount of the resistance which the rising or falling of the mer- 

 cury in the tube has to contend with ; and this is all extremely 

 little, so little that the index moves by the unequal action of 

 the wind during a light gale, and is put into a state of oscil- 

 lation of some considerable duration by the mere opening of 

 the door of a room. These pulleys measure very nearly one 

 inch in circumference, so that if the mercui'y moves an inch 

 the index is carried once round the brass circle, and hence one 



division thereon corresponds to fuooth of an inch, a correction 

 being made on the pulley according to the relative capacity of 

 the tube and the bulb. 



The index is made in three parts, of light straws, a centre 

 piece and two extreme pieces inserted into it : one extremity 

 is cut after the manner of a pen to a somewhat short and very 

 fine point, which is turned edgeways. The whole is carefully 

 equipoised by a short piece of straw sliding on one of the ex- 

 treme pieces, so that when attached to the axis it takes indif- 

 ferently any position in the circle, and, consequently, follows 

 exactly the movement of the mercury. 



