* 
J. Lovering on Velocity of Light and the Sun’s Distance. 169 
tance by observation, the others can be computed from this law. 
As the solar parallax is only about eight seconds, and an error 
of one-tenth of a second includes an error of more than a million 
of miles in the sun’s distance, he takes advantage of the law of 
Kepler, and selects a planet which comes occasionally nearer to 
the earth than the sun. The choice lies between Venus at infe- ° 
fore, may be nearer to the earth than Mars, and the parallax 
more favorable. But Venus cannot be seen at conjunction ex- 
cept when its latitude is so small that a transit across the sun’s 
dise occurs. Then the two observers refer its place not to a star 
but to the sun, and the quantity they determine is the difference 
of parallax between Venus and the sun; which will vary from 
about 21" to 25”. Moreover, the difference of parallax is meas- 
ured, not directly, but through the influence it produces on the 
duration of the transit at the two stations: and, therefore, upon 
a greatly enlarged scale. 
What are the results which have been obtained: 1st, by ob- 
servations of the transits of Venus, and 2d, by observations on 
Mars at opposition ? 
1. Only two transits of Venus have occurred since the time 
when the sagacious Dr. Halley invoked the attention of posterity 
to these rare astronomical events as pregnant with the grandest 
results to science; viz: those of 1761 and 1769. The astrono- 
mers of the last century did not neglect the charge which Halley 
consigned to them. The transit of 1769 was eminently favora- 
ble, offering a chance which comes only once in a millenium, as 
Professor Winthrop happily explained in his lectures on the 
last transits. 
Whatever verdict posterity shall pronounce on the deductions 
from the observations then made, they will never, says Encke, 
reproach astronomers or governments with negligence or want 
of appreciation towards this golden opportunity. The solar 
parkibex which Encke deduced from an elaborate discussion 
all the observations, fifty years after they were made, is 857116. 
‘his corresponds to a solar distance of 95,360,000 statute miles. 
Although transits of Venus will take place in 1874 and 1882, 
and astronomers already begin to talk of preparing for them, I 
have the authority of Kncke for declaring that, in compariso 
With that of 1769, the next two transits will be so unfavorable 
that nothing short of perfection in the construction of instru- 
_Ments, and in the art of observing, can compensate for the natu- 
Am. Jour. Scr.—Seconp Serres, Vou. XXXVI, No. 107.—Szpr., 1863, 
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