On the MOTION of LIGHT. 89 



to /3, the hole B will have moved over By, which is three- 

 fourths of B/3 ; therefore an eye, placed behind the hole B, 

 will not fee the object through the fights of the inftrument, but 

 will fee an object lying to the right hand of it, having its angu- 

 lar diftance equal to yA/3 ; or, if the inftrument, inftead of ha- 

 ving a hole A for the anterior fight, has two fine wires crofting 

 each other in A, the object will appear on the oppofite fide of 

 their interfection from that towards which the earth is moving. 



From this reafoning, Mr Boscovich concludes, that if this 

 inftrument be placed at the earth's equator on the day of the 

 winter's folitice, and the index be directed to the real place of 

 an object due fouth of it, the object will not appear at the in- 

 terfection of the crofs wires of the anterior fight, but, at noon, 

 will appear 5" to the eaft of that interfection, at fix o'clock in 

 the evening, it will appear 5" above it ', at midnight, it will 

 appear 5" to the weftward of it ; and at fix o'clock next morning, 

 it will appear 5" below it. The object will, therefore, appear to 

 defcribe a circle round the interfection of the crofs wires in 

 twenty-four hours, the diameter of which will fubtend an an- 

 gle of 10". In other pofitions of the index, and other days of 

 the year, the apparent motion of the object will be different ', 

 but it will never appear in its true place, except in thofe in- 

 flants that the fiducial line of the inftrument happens to be pa- 

 rallel to the line of the earth's motion at that time. 



Mr Boscovich proceeds to fhow that, if the theodolite has 

 a common aftronomical telefcope, inftead of plane fights, the 

 appearances will be precifely fimilar. If the index be directed 

 to the real place of an object, the object will be feen at the in- 

 terfection of the crofs wires of the eye-piece ; and if the inftru- 

 ment be firmly fixed in its pofition, the object will always ap- 

 pear at this interfection, notwithftanding the motion of the 

 earth. But if the telefcope be filled with water, and be di- 

 rected to the real place of an object fituated as above mentioned, 

 on the noon of the winter's folftice, the object will appear 5" to 



Vol. II. m the 



