[ 562 ] 
and the leaft motion of the telefcope, whilft the ob- 
ferver is turning his eye from one wire to the other, 
muft oblige him to repeat the obfervation; whereas, 
by this method, the contadt of the edges of the images 
is not at all affedted by the motion of the telefcope. 
Whence the comparifon of this micrometer, with the 
common fort, in this refpedt, Hands thus : The one 
requires great fieadinefs in the telefcope, but yet it is 
applicable to none, but fuch as are very difficult to 
keep Heady ; the other does not require fuch fteadi- 
nefs, tho’ it is applicable to fhort telefcopes, ; which 
are eafily managed. 
Thefe advantages not only add to the certainty of 
the obfervation, but adifi vaHly in the expedition ; 
for an obferver may make twenty obfervations, in this 
way, where he could fcarce, with much fatigue, be 
fure of one with the common micrometer. Ex- 
pedition in making obfervations, muH be allowed a 
very great advantage, in this climate, where the un- 
certainty of the weather renders afironomical obfer- 
vations fo precarious, that no opportunities, even the 
mofi tranfient, fhould be let Hip. An infiance of this 
was given in to the Roval Society, in an account of 
the eclipfe of the fun lafi Odtober. 
As the motion of the telefcope gives the obferver no 
great inconvenience, in this method > neither does the 
motion of the objedt at all difiurb his obfervation (I 
mean fuch a motion, as that of the heavens is,). This 
gives him leave to take the diameter of a planet, in 
any diredtion ; or the difiance between two fiars or 
planets, let their fituation be how it will ; in which 
refpedt the common micrometer is abfolutely de- 
fedtive ; as it can give no angles, but fuch as are per- 
pendicular 
2 
