a  —— 
me 
3 
79 
the planet’s parallax at once, —the element directly deduci- 
ble bearing to the solar parallax the following ratios : 
At transits of Venus, 2.57 
At oppositions of Mars, 1.92 on the average. 
“ “ee “ee ce 
2.74 in extremely favorable cases. 
Inf. conjunctions of Venus, 3.57 i 
Stationary positions of Venus, 2.94 
Thus the observations of Venus promised to yield a 
better determination of the solar parallax than any opposi- 
tions of Mars; and those at the stationary or turning points 
of her apparent path, a result surpassing in accuracy that 
from the average of these oppositions by about $ of its 
whole amount. The natural objection that the sonjenciien 
of Venus must be observed by day, thus dispensing with 
the advantage of micrometer comparisons, and requiring 
meridian observations at midday, was recognized by Dr. 
Gerling, but the excellence and power of the newer meridian 
instruments were cited as compensating for this serious 
disadvantage. The observations during the stationary 
_ period were, however, chiefly urged. 
Before the publication of this memoir, in which the sub- 
ject was discussed at very considerable length, Dr. Gerling 
had, in April of the same year, (1847,) written to Gilliss, in 
acknowledgement of his volume of observations, and, in his 
letter of thanks, gave some account of his proposition. 
“I am of opinion,” said he, “that astronomers act unwisely 
in considering the solar parallax deduced from the transits 
of Venus in 1761 and 1769 sufficiently correct, and do not 
avail themselves of more modern methods of observation, 
for the purpose of gradually acquiring more accurate knowl- 
edge of it. It is true, indeed, that the oppositions of Mars 
were long ago proposed for this purpose ; but I am not 
‘aware that any effective use has been made of them since 
