138 Astronomical and Magnetical Observatiotis. [Aug. 



they differ from one another. Unless we deceive ourselves 

 respecting the justice of those results which we ourselves have 

 obtained, it will be seen, by what follows, how far they all 

 deviate from the truth. 



( To be continued.) 



Article XII. 



Astronomical and Magnet ical Observations at Hackney Wick, 



By Col. Beaufoy. 



If the wires of a transit instrument be placed at unequal 

 distances from the centre wire, and from each other, the fol- 

 lowing method may be used to correct the mean of the times of 

 the sun or star passing the lateral wires, (i. e.) those on each side 

 of the middle one. 



Let a, b, c, d, represent the equatorial intervals, or the time 

 that a star in the equator takes to pass from wire to wire ; and 

 suppose x to be the time that the star passes the first or eastern- 

 most wire ; then x + a is the time of passing the second wire ; 

 x + a + b, the time of passing the third wire; x + a + b -f c, 

 the time of passing the fourth wire ; and x+a + b + c + d, 

 the time of passing the fifth or last wire. 



If the wires had been equi- distant, it is evident half the sum 

 of the first and fifth, or second and fourth, would have been 

 equal to x + a + b, or i of the times of passing all the wires ; 

 therefore the differences between each, and x, a, b, will be the 

 required corrections. 



Suppose the equatorial intervals of a, b, c, and d, to be as 

 follows: a = 25'33", b = 25-87", c = 25'87", d = 25- 17", 

 then x + 50-40" is equal to x + a + b, the time of passing 

 the middle wire ; but a mean of all the five wires is a + 50*78": 

 therefore the correction is 0*38" to be subtracted. 



Again, the sum of the first and fifth wires is 2 x 4- 101 -93", 

 its half is x + 50-97", a °d the correction 0*57" to be sub- 

 tracted. Half of the sum of the second and fourth wires is 

 x + 50-80", the correction 0-40" to be subtracted. 



a = 25-33" 



b = 25-07 



c = 25-87 



d = 25-G7 



x Time of passing the 1st wire. 



x + 25-33" Ditto 2d 



x + 50-40 Ditto 3d 



+ 76-27 Ditto 4th 



x 



x + 101-93 Ditto 5th 



