New-York tribune., 1875-03-08

the Americans obtained some photographs Is by
no means conclusive as to the number or value OF
the latter, since the American system of obtaining
sun pictures essentially differs from the English, and
at several stations has been far more successful.
THE USE TO BE MADE OF PHOTOGRAPHS.
The value of photographs of the sun taken during
the transit is yet to be determined The object
sought to determine the precise track which Venus
took when she crossed the suns disk. Astronomers
know very accurately the law of Venus's motion
they may be said to know absolutely for any given
small distance Of her orbit. As each photograph of
the sun was taken with Venus upon its disk just at
the instant the plate exposed 10 take the photo
graph the time was recorded by break in an cleo
tric circuit which registered itself 1t necessary
afterward to develop and fix the picture.
The next step will be to put 1t
under the microscopes Of the elaborate measuring
apparatus of the Naval Observatory, and determine
TO the last degree the exact distance between the
center of Venus and the center OF the sun on each
plate. This is done by measuring the distance OF
the suns center and the center of Venus from
fixed reference line the plate the line being In
reality the photograph of the fine platinum wire
which held a plumb bob swinging III the cen
ter Of the iron pier Supporting the sensitized plate
Thus will be obtained for each picture the approx
mate position of Venus at the Instant OF time which
the electric recorded as Delng WHICH,
exposure was made. Having a series of pictures Of
Venus upon the sun from each station, 1t Will be
possible to plot the track described by the
planet. The opinion has been advanceo that II
the observations of actual contact not made the
success of the American work has not been insured
OF course it would be better to have had complete
observations but not necessarily because the actual
contacts needed TO give the path of Venus The
law of Venus's motion being we'll known for short
distances such crossing the suns disk, If only
a very few accurate positions OF the planet during
the transit are obtained it is not difficult to compute
back to the beginning and forward to the end of the
transit with smaller probable error than there is
in the single observation of q contact where many
difficulties come in to battle the observer. In the
present transit there discrepancies of ten seconds
and more in the estimates of contacts seen by
the eye, and while in many of the photo
graphs there is not that sharpness Of the
suns and Venuss disks which seems desirable
still number of the pictures used together Will
enable the astronomers to plot track at any single
station. which must be close approximation. Our
American parties have trusted mainly to the photo
graphic records it is true that in doing so we have
as the Germans have phrased it, trusted to an un-
tried ally 1t cannot be certainly known yet
whether the films on the glass plates will shrink or
deterorate in any way before they reach Washing
ton They must be transported many thousands of
miles through VARYING climatic conditions. All that
can be said at present that if the photographs meet
the expectations which have been based upon their
numbers and apparent usefulness there is reason TO
believe that the American work as whole, COM
pared with that of other nations is the most effective
OBSERVATIONS OF ENGLISH PARTIES.
The English were less fortunate In New-Zealand.
From Christ Church. n. L.. their reports stated that
the observation of the ngress spoiled by clouds
which prevented any measures being taken with
the double image mIerometer prior to the internal
contact and entirely hid the planet from view
few seconds before the contact itself Photographs
near the contact were impossible About IA
minutes after the contact the sun appeared
a little more brightly at intervals A few
micrometrical measurements were made during
some eight or ten minutes bur these Will be
of little A few photographs most of them
Indifferent were secured. At o'clock the sun be-
came hidden and was not seen at all from halfpast
o'clock until 10 minutes after the end of the
transit when 1t shone brightly till nearly sunset.
The five sub-stations at GrahamstOwn, Auckland.
Wellington, Dunedin and Naseby met with the same
bad fortune both at ingress and egress. The Aus
tralan stations gave better results. The Royal
Observatory at Melbourne reported numerous pho
tographs of ingress with good telescope observa-
tiOns of the first internal contact. Obser-
vations were made and photographs obtained at
WoOdford, Eden. GOuldburn, Brisbane, Adelaide,
and Windsor Altogether the English secured $50
photographs of the whole sun, and $50 Of Venus
entering upon and leaving the SUMS disk. But for
comparison with observations at the north or the
west, these records are not of the highest value.
recent writer in Nature, referring to these
stations says: 1t is doubtful if all the resources
Of first class fixed observatory, possessed by Mr.
Elery at Melbourne, will make up for his compara-
lively poor position" The most successful southern
observations of the English were made at Sydney.
several hundred miles north OF Melbourne and
therefore yet more undesirable staton, so far as
the comparative displacement of Venus with refer
ence TO northern stations is concerned.
English parties sent to the Sandwich Islands,
under Capt. Tupman; 10 the photograph station at
Roorkee in India, under Col. Tenant lin the
jurisdiction of the Indian Government to Alexan-
dra in Egypt; to Rodriguez private party went to
Mauritus, under Lord Lindsay; and lastly party
was sent TO Kerguelen Island. The chief dependence
Of the English is on the method known Delisles; ;
the accuracy of this method depending, roughly
speaking upon securing observations from two
points whose longitude shall be known to within
a single second of time, these points differ
ing as much in longitude as may be prac-
ticable Hence if the English secured good
observations at the Sandwich Islands, and also at
one of their western stations, they have the ele-
meuts for the solution of the problem. At the
Sandwich Islands the atmospheric conditions were
favorable for the observations; $50 measures of
cusps and limbs, and 60 photographs were obtained
of the latter, 90 came ont blurred. The day was
fautless and,Yenus could be on the disk Of the
sun Just as the sun sank below the horizon From
Lord Lindsays party at Mauritius the news was,
on the whole gratfyng the last half Of the transit
was quite satisfactory. Good photographs, meas