a Lamp-Micrometer . ffty 
the fame height and diftance meafuring from the right fide. 
The wick of the flame confifts only of a Angle very thin lamp-* 
cotton, thread ; for the fmalleft flame being fufficient it is eafier 
to keep it burning in fo confined a place. In the top of each 
lamp muft be a little flit, lengthways, and* alfo a fmall open- 
ing in one fide near the upper part, to permit air enough to cir- 
culate to- feed the flame. To prevent every reflexion of light, 
the fide opening, of the lamp a fhould be to the right, and that 
of the lamp b to. the left. In the Aiding door of each lamp is 
made a fmall hole with the point of a very fine needle juft op- 
pofite the place where the wicks are burning, fo that when the 
Aiders are fliut down, and every thing dark, nothing (hall be 
feen but two fine lucid point3 of the fize of two ftars of the third 
or fourth magnitude- The lamp a is placed fo that its lucid point 
may be in the center, of the circular board where it remains fixed. 
The lamp b is hung to the little Aider which moves in the rab- 
bet of the arm, fo that its lucid point, in a horizontal pofition 
of the arm, may be on a level with the lucid point in the cen- 
ter. The moveable lamp is fufpended upon a piece of brafs- 
faftened to. the Aider by a pin exa&ly behind the flame upon 
which it moves as a pivot. The lamp-is balanced at the bottom 
by a leaden weight, fo as always to remain upright, when the 
arm is> either lifted above, or deprefled below, the horizontal' 
pofltion. The double-jointed handles rD, eP, confift of light 
deal rods, ten feet long, and the loweft of them may have divi- 
flons, marked upon it near the end P, exprefiing exa&ly the 
diftance from, the central lucid point in feet, inches, and 
tenths. 
From this conftru<ftion we fee, that a perfon at a diftance of 
ten feet may govern the two lucid points, fo as to bring them 
into any required pofition fouth or north preceding or following; 
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