78 
Survey the entire series of moon-culminations previously 
observed and published by him. Fifteen manuscript folio 
volumes in the archives of the Survey contain this valuable 
work, the subsequent discussion of which by Walker, and 
still later by Peirce, led to the investigations by these geom- 
eters into the relative accuracy of Gilliss’s observations, con- 
_ cerning which I have already spoken. 
In May, 1847, Dr. Gerling, the eminent mathematician of 
Marburg, published a memoir, calling the attention of astron- 
omers to the fact that the universally adopted value of the 
solar parallax depended solely upon observations of the tran- 
sits of Venus in 1761 and 1769; and that, although the 
materials afforded by the observations then made had doubt- 
less been exhausted by the labor and skill with which Encke 
had deduced the value since adopted by astronomers, yet & 
constant so important_as this, which affords directly or in- 
directly the sole unit for the determination of all celestial 
distances, should not be subject to the possible uncertainties 
of any one method. Especially was it unfortunate that the. 
only method employed depended upon a phenomenon which 
recurred, doubly to be sure, yet only at intervals of more 
than a century ; and which would not again take place until 
after the lapse of more than a quarter of that period. 
The combination of observations of Mars at opposition, 
made from _terrestrial stations widely differing in latitude, 
had been frequently suggested ; but Dr. Gerling advocated 
especially the similar employment of observations of Venus 
at inferior conjunction, and especially when at, or near, the 
Stationary points, and of oppositions of Mars. His convie- 
ae focal in favor of this method rested principally on the con- 
ose a whereas, in transits of Venus, the —— 
. ined is the difference between the of 
the net once that of the sun,—the other methods yield 
