1875./ Notices of Books. 259 
and conclusions which move in a narrow circle, or a host of 
propositions swollen out with tautological excesses, and like a 
windbag ready to burst on the application of the least pressure. 
And here we venture to think the philosopher of science has the 
advantage of the philosopher of morals; for the former can 
scarcely argue in a circle when he is confronted at every step by 
facts wrought out of nature by legitimate experiment and exact 
induction. 
The Transit of Venus: its Meaning and Use. By T. H. Bupp, 
F.R.A.S. London: Longmans, Green, and Co. 1875. 
In this clever and concise little pamphlet the author tells us, in 
plain untechnical language, all that a general reader interested 
in science, but not specially an astronomer, can require to 
know. We will briefly condense the main facts of the case. 
The transit of Venus means, we all know, the passage of Venus 
across the face of the sun; and since the moon eclipses the sun 
under like circumstances, and Venus has thrice the diameter of 
the moon, we may reasonably enquire why the sun is not 
eclipsed during the transit of Venus. But the fact that Venus 
is far more distant from us than the moon converts what would 
be an eclipse into a transit. The transit occurs twice in 
116 years, with an interval of 8 years between two successive 
transits in that period, e.g., in 1761 and 1769, and again in 
1874 and 1882. The long interval is due to the fact that 
the plane of the orbit of Venus is inclined to the plane of 
the earth’s orbit round the sun. The exact observation of 
the transit of Venus enables astronomers to ascertain our 
distance from the sun; and as it is believed that an error 
-of three millions of miles exists in our present estimate of 
that distance, the necessity of a new determination is obvious. 
There must always be some error, but repeated observations 
will reduce it toa minimum. The Author points out and illus- 
trates by a diagram how, by means of a theodolite and a simple 
trigonometrical method, it is possible to measure an inaccessible 
object, and how this method may be applied to the sun when a 
suitable base line has been obtained. The greatest possible 
distance between two observers on the earth’s surface—that is, 
the greatest possible base line on one and the same meridian—is 
something less than 7000 miles, because the polar regions are 
inaccessible. The observation of the different positions of 
Venus when crossing the sun’s face, by two distant observers, 
enables us to ascertain the diameter of the sun, and by com- 
paring his apparent diameter with his real diameter we can 
ascertain his distance from us in the same way that we can 
determine the distance of a church steeple if the height be 
known. 
