Lovering and Bond on Magnetic Observations at Cambridge. 5 
continued from that time, which by their comparison afford satisfactory 
evidence of the stability of the pillars on which the instrument rests. 
F Fis the direction of the astronomical meridian and intersects 
Blue Hill, in Milton, west of its summit. A firm and substantial 
meridian mark has been erected on that hill, consisting of a tower 
of round and substantial masonry, thirteen feet in diameter at the 
base, seventeen feet high above the ground and nine feet in diame- 
ter at the top. On this is placed a mark seven feet high, of the 
shape of the rhomb, with its larger axis perpendicular to the horizon. 
By this means the central vertical wire of the Transit instrument is 
put in the meridian. It appears from Mr. Borden’s State Survey 
that the distance between the mark and the instrument is 58,520.5 
feet. A brick house on the meridian line, about a mile from the 
Transit-instrument, affords a convenient though less accurate refer- 
ence, when the state of the atmosphere does not allow the distant 
mark to be distinctly seen. 
A short abstract from the astronomical records of the Observatory 
is annexed, to show the state of the Transit-instrument and the 
confidence that may be reposed in the accuracy of the time used in 
the magnetic observations. The equatorial intervals of the wires of 
the Transit-instrument, as deduced from a large number of observa- 
tions, are thus: 
iS) 
From Ist wire to the mid-wire : 33.96 
From mid-wire to 5th ; 5 * 33.94 
From 2d to mid-wire 3 ‘ : 16.88 
From mid-wire to 4th : : . 16.86 
The following extract from the Transit-book includes all the standard 
stars whose transits were observed from July 10th to July 25th, 
1840. Ina few cases, an observation has been rejected because 
one of the five wires was accidentally missed. The introduction 
of such stars would vitiate the general comparison. 
