Seven-feet Transit Instrument. 425 
principal parts of the instrument are united to the sphere, 
but figures 8 and 9 of Plate XVIII. will illustrate a description 
of what is hitherto peculiar to the Greenwich transit instru- 
ment and mine. 
The ends of all the four cones, where they join the sphere, 
are strengthened by circular pieces of cast brass ; these pieces 
extend full three inches into the lengths of their respective 
cones, into which they are soldered and pinned ; they are 
turned concave in front, so as to fit the surface of the sphere, 
into which they are rabbeted, and serve to keep the opposite 
branches of the axis and telescope straight, and at right angles 
with each other. To these brass pieces are attached broad 
and strong rings, for the reception of the screws which bind 
the whole together. 
The four branches of the axis and telescope are solely 
united, by what Mr. Troughton calls, tension bars ; these 
bars pass through the sphere, six of them in the direction 
of the axis, and four in that of the telescope. They are 
arranged at equal distances between corresponding parts, 
care being taken that those of the axis do not obstruct the 
rays of the object-glass, and that the illuminator is not sha- 
dowed by those of the telescope. The tension bars screw 
into the rings of the brass pieces above described ; they have 
at one end a fine screw, and at the other a coarse one ; the 
fine one is made about twice as long as under other circum- 
stances would be required : and there are holes in the sphere 
at proper distances, through which the bars can pass freely. 
To connect these various parts, let the fine screw ends of 
the six bars of the axis be screwed into their proper rings as 
far as they will go ; then pass the bars through the holes in 
the sphere, and pressing the cone home upon the rabbet 
