Ill 



general result gave 121",1, for the equatorial interval of the 

 fifteen revolutions. This interval, reduced to space, made 

 each revolution of the figured head — to 2' and 1" of mea- 

 sure; and, of course, each of the fifty sub-divisions = to 

 2",42 nearly of an arch of the equator. In making the 

 subsequent measures. of the sun's diameter, or that of any 

 other celestial arci the measure was always finished, by 

 moving the wire, in the direction in which the fifteen re- 

 volutions were originally made. The advantage of this mi- 

 crometer is principally this: that, in adjusting the telescope, 

 and the xnierometer wires, to distinct vision, no alteration 

 is made, by the difference of the conformation of the eye, 

 ori of focal distance, tliat suits that of the observer, in the 

 value o^ the arc to be measured. The principal defects 

 of it are : the dififtculty of judging accurately of bisections, 

 or contacts of the fine wires, by the limbs to be measured; 

 and the impossibility of observing any diameter, except 

 the one perpendicular to the equator. 



The object-glass micrometer is an instrument, now so 

 familiar to every person conversant in the use of astrono- 

 mical instruments, that it is only necessary to say, that 

 mine was made, and adapted to a triple object-glass achro- 

 matic telescope, of 42 inches focal distance, by Mr. Dol- 

 land, and its scale very carefully verified by himself; and 

 that the scale is, as visual, divided into inches, 10""' 20""' 

 and vernier divisions: that, when it is applied, it lengthens 

 the focal distance of the telescope about 6 inches; thus 

 making it 48 inches, or 4 feet focal distance. 



p2 The 



