aid Mr. atwood’s Theory for the Menfuratlon 
differing in this from Mr. hadley’s conftruCtion, in which 
the angle obferved is equal to double the angle defcribed by the 
moveable radius from the initial point of the arc O. While 
therefore the moveable fpeculum C is carried round by the ra- 
dius CP in the plane of motion according to the new conftruc- 
tion juft defcribed, the image of Emoves with an angular velocity 
juft equal to that of the radius, the motion of the image being, 
according to Mr. Hadley’s invention, always greater than that 
of the radius in the proportion of 2 to 1. adly, The angles 
of incidence and reflection on both furfaces are conftantly the 
fame, being equal to 45 0 . 3dly, BI (art. 13.) being always 
lofs than 90% obfervations by two reflections may be taken all 
round the circle, that is, angles of any magnitude mav be 
meafured by this conftruCtion. It will not be difficult in prac- 
tice to regulate the inclination, of the plane refleCtors to the 
plane of motion, with the other given quantities to their true 
magnitude. Let the refleCtors B and C be brought parallel 
when the index or radius CP is dire&ed to O, being the initial 
point of the arc OP : in order to examine whether the fixed 
plane' of reflection BFD be coincident with the primitive fecon- 
dary KO, it is only neceflary to obferve the angle fubtended by 
two given objeCts when the index CP is on the different fides of 
the initial point O : if the index be directed to unequal diftances 
from that point at the times of obfervation, a correction is re- 
quired (art. 11.). To examine whether the inclination of the 
refleCtors to the plane of motion be exaCtly 45% let the index 
CP be directed to t8o° : if the inclination of the refleCtors to 
each other be not then = 90°, a correction muft be applied. It 
will be known whether the inclination of the refleCtors to each 
other be =90°, by obferving the two oppofite horizons at fea, 
and at land by various obvious methods. Thefe examinations 
are 
