160 
W. Beebe — The Comet of 17 71. 
present clay, but the declinations are less so, differences of declination 
of 2° to 3° having been measured. 
b. Forty-eight by Messier, the discoverer, at Paris, differential 
measurements made with a two-foot finder of a larger instrument, 
furnished with a rhomboidal reticle of his own construction. It is 
referred to by .Messier as a “ machine parallactiquef and apparently 
had the same mounting as Maskelyn’s. The comparison stars and the 
differences of Rt. Ascension and declination are published in the 
Histoire de I? Acadamie for 1777. The clock errors and rate are 
not given and the comparisons in R. A. are only to even seconds of 
time. 
c. Forty-seven at Kremsmunster, Rouen and Stockholm. The ori- 
ginal observations are not preserved. The results are published by 
Messier in connection with his own. Nineteen at Stockholm are 
accompanied by a list of comparison stars, but the differential mea- 
sures are not given. All forty-seven cover the same period as 
Messier’s, and not being, like his, susceptible of independent reduction, 
are of comparatively little value. There is no record of the instru- 
ments with which they were made. 
d. Sixty-three at Marseilles by St. Jacque de Sylvabelle, published 
by Messier. Sylvabelle made two sets of measurements, direct 
and differential. Messier says that his differential measurements 
were as many as 400 or 500, but that for want of a catalogue of the 
comparison stars they were never reduced but pi*eserved in the 
original manuscript. When in 1820 Encke computed the orbit, he 
visited Marseilles, and made thorough search for them at the observa- 
tory, but without success. Their loss is much to be regretted, since 
the climate of Marseilles allowed observation to continue up to July 
17, whereas the latest elsewhere is on June 9, one of Messier’s on 
June 19 being only an approximation. The succeeding computa- 
tions show that the later observations are of decisive influence, while 
Sylvabelle’s direct measurements, which are the sixty-three pub- 
lished, are inaccurate in a high degree, as we should expect from 
the account we have of them. 
Messier says that lacking a catalogue of stars, Sylvabelle “ made 
use of the R. A. and Reel, circles of the parallactic machine” of his 
eight-foot telescope. This must mean that bringing the comet into 
the center of the field of the finder of the telescope in which 
he made the differential measurements, he then read oft' the circles 
which guided the motion of the telescope on its portable stand, and 
entered the results as the comet’s position. In such observations the 
