New-York tribune., 1874-12-05

graphs during the transit and afterward making
measurements from them, by means of microscopes.
The foregoing explanation Of the direct method suf-
fcicntly indicates how the Photographic measures
can be utilized. The American observers will pay
special attention to photogaphing the sun during
the transit and recording the ilstants at which the
pictures are taken telescope 40 feet in length, TO
be laid in horizontal position, is to be used in this
operation, simple and ingenious instruments having
been devised for rejecting the moving image Of the
sum- and carrying ont the other details of the pro-
cess. The great success which Americans have met
with in applying the photograph to astronomy Is
largely due to the perseverance and skill OF our
townsman, Mr. Rutherfurd, who has freely devoted
time and fortune to this method of research.
UNIIrY's METHOD DETERMINING DISTANCE.
The methods already described indicate the use
that can be made Of observations at the beginning
of the transit and at its middle. Hallcys method
so-calledJ differs from these by requiring knovl-
edge Of the end as well rs the beginning of the
transit in order 70 ascertain the whole time taken in
the passage of Venus across the sun. Reyertlng to
the preceding figure it is Obvious that the observer
at the more southern station, b, sees Venus for
shorter time the surface Of the sun than the
observer at A. The proportonate length of
the chord ID f to the chord L G. can there
fore be calculated Now the apparent
at the time of the transit las been long since
tanned by careful observation, and it is known
exactly to an exceedingly minute fraction how
long it would take her at this time TO pass such
a space as the apparent diameter of the sun. Each
Of these chords ID +, c. can therefore be COM
pared with the diameter Of the sun. If the
path of Venus in this transit were exactly across
the center of the sum- she would make the transit of
the whole diameter of the sun III hours 53 minutes
and 30 seconds. If the most southerly observer on
the earth finds that the transit "a seen by him takes
hours 81 minutes and 40 seconds Il. E.. half the time
for crossing the diameter it is manifest that the
line or chord ID f il half the length of the suns
diameter. II is very easy to put such chord in Its
proper place in circle. When farmers boy gets
pitchfOrk aud makes circles with it on a barn
door, he is very likely to amuse himself by draw
ing three semi-crcles within the circle which
start from its edge and pass through Its center,
striking the edge of the circle at six equally distant
points these points are distant from each other
just half the diameter of the circle, for the pitch
fork which makes the circle can give this distance
and no other line drawn from any to the
next one of six points that equally divide a circle is
the chord which we are supposing that southern
observer sees the path Of the transit of Venus.
little geOmetrcal calculation which readers may
make for themselves or study ont by reference to a
table of snes OF measure with compass and scale on
a piece of paper will show that the center CT this
chord is distant from the center of the circle pretty
nearly seventeen-fOrteths Of the diameter. This Is
the position supposed for this chord in our previous
explanation of the direct method The position
of another chord G. is obtained by the other OBJ
server at A. and q method by which the distance OF
the sun NAV be calculated with these facts In our
possession-the position of two chords on the suns
surface and their space apart in miles-has already
been gwen.
1t is perhaps desirable to say again that the frac-
tions and figures given in this article are mere APP
proximations and would be of no value to an as
tronomer who must work with the incest accuracy.
But the reader may amuse himself by testing one OF
the suppostions of this article by the large diagram
of the sun Venus and the path of the transit which
heads this article and is drawn to scale The upper
line of the central path is nearly the path OF the
center of Venus as seen by the most sOuthery OBJ
server Ii was assumed in this article that the most
southern observer would path whose length
would be about half the suns diameter Ii Is easy
to measure on the diagram the distance from the
suns center to It's circumference aud compare with
the upper line of the central path OF venus Inc
lengths will be found somewhat nearly 10 cor
respond the difference being no greater thah the
fractions thrown aside in the figures used in the
foregoing explanations That diagram aud the map
of the hemispheres are reduced from the drawings
Of Mr. Richard A. Proctor. TO whose lucid exposition
of the subject all recent students of the problems OF
the transit are deeply indebted
A SIMPLE EXPLANATION OF TIE PARAllN'
Astronomers long ago fell into bad habit wnlch
neither adds TO their own facilities for calculation
nor TO those OF the public for understanding them
Following the method of the ancients. they con-
tnue to divide the circumference of circle into $40
degrees then each of these degrees into % minuts
then each Of these minutes into 20 seconds IL would
be far simpler to divide the circumference into dccf