178 
SCIENTIFIC SUMMARY. 
ASTRONOMY. 
^RANSITS of Venus in 1874 and 1882. — If the next pair of transits of 
Venus should fail to afford a satisfactory determination of the sun’s 
distance, it will not be for want of due care on the part of our astronomers 
to prepare for the necessary observations. So far back as 1857 the Astro- 
nomer Royal called the attention of the scientific world to the requirements 
of each transit. He pointed out that the method wdiich was pursued in, 
1761 and 1769 will be wholly inapplicable in 1874, and is embarrassed in 
1882 with the difficulty of finding a proper station on the almost unknown 
Antarctic Continent. The recent publication of Leverrier’s new tables of 
Venus, and of calculations founded upon them by the indefatigable Mr. 
Hind, have induced the Astronomer Royal to re-examine the whole subject. 
He has come to the conclusion that it will be unsafe to trust exclusively to 
the chance of securing obseiTations on the southern continent in 1882 ; and 
that it will be desirable to make observations, both in 1882 and 1874, 
directed specially to the determination of the acceleration and retardation of 
the planet’s ingress and egress, as affected by parallax. In order to under- 
stand the principle on which this method is founded, let the reader suppose 
himself placed at that point of the sun’s surface where (as seen from the 
earth) first contact takes place, and that he watches from thence the passage 
of Venus across the earth. It is clear Venus would appear to him larger 
than the earth ; the disc of Venus would come up to the edge of the earth’s 
disc at a certain point and sweep across that disc, until at a point almost 
exactly opposite to the former the occultation would be complete. The 
proce.ss would last about ten minutes : the first point reached would clearly 
be that part of the earth’s surface at which the ingress of Venus would take 
place earliest; the second would be the part where the ingress would take 
place latest. And it is obvious that if two observers were placed at these 
spots, one at eacli, and severally timed the moment of apparent ingress, the 
knowledge of the exact interval would be available as a means of determin- 
ing the sun’s distance. Similar considerations apply to the egress. In 
practice, the pf»ints we have named would not be available, because the sun, 
as seen from them, would be upon the horizon at the moment of ingress ; 
but spots could be so chosen a.s to give a sufficiently large interval, and yet 
to allow the sun to be well raised above the horizon at the moment of 
ingress. So also for the egress. 
