the 'Trigonometrical Operation . 14 1 
only ufed for terreftrial objects, it requires but a fmall elevation 
or depreflion, and therefore is only fupplied with a fhort axis 
of feventeen inches in length, fupported by braces attached to 
the feet. The eye end of this telefcope is purpofely made hea- 
vier than the objedt end ; and refting on an horizontal arm, 
that is raifed or deprefled by rack-work, it is thereby readily 
brought to bear, and remain very fteadily, upon its -objedt. 
The rack-work may be leen in the view of the inftrument, 
and alfo on the left fide of the right hand fedtion in Plate IV. 
But there is a fmall horizontal motion that can be given to the 
right hand end of the axis of this telefcope, which is effected 
by means of a handle inferted through the vacancy of the ba- 
luftrades, and placed on a dovetail at E, which could not be 
fhewn in the plate. Thus the inftrument being nicely levelled, 
the upper telefcope at zero, and likewife on its objedt, the 
lower telefcope, by the help of this adjuftment, is brought 
accurately to the fame objedt, fuppofed to be the point of com- 
mencement, or that from which angles are meafured. 
By referring to Plates III. and IV. and likewife to the fedtion 
on the left fide of Plate V. it will be feen, that a horizontal 
bar extends acrofs the top of the vertical axis, fupported by 
two fide braces that fpring from the cone, about one-third of 
its height above the plane of the inftrument. The horizontal 
bar carries the Y’s or fupports, in which the pivots of the 
upper telefcope move. They are of fuch height as to permit a 
femicircle of fix inches radius, attached to the axis of the 
tranfit, to pafs freely, and confequently the telefcope to be 
diredted to the fun or ftars in high elevations, but not to be 
brought to the zenith. The arc of excefs of the femicircle 
likewife admits of feveral degrees of depreflion being mea- 
fured thereon. 
Art* 
