Re M&D in SA

[Breytenbach, C T]

A few list members complained that they couldn't open the attachment to
my message of last Friday. Sorry about that.  Here goes again.

Kerneels Breytenbach

Article from the Montly Notes of the Astronomical Society of South
Africa, November 1951, pp. 99-102

MASON AND DIXON AT THE CAPE
 by
Theodore MacKenzie F.R.A.S.


In Astronomy and Geodetics the names of Charles Mason and Jeremiah Dixon
are inseparably linked together. They were colleagues in making
observations of the Transits of Venus in 1761 in 1769, in the survey of
the famous  Mason and Dixon Line  in America and in the measurement of
the length of a degre of latitude also in America. How they came to be
at the Cape in 1761 is almost what Shakespeare would call a
tragical-comical-historical-pastorial  story which I regret must be
considerably curtailed in this talk.
	Though Gregory was the first to point out that transit of Venus over
the Sun+s disc afforded a means of ascertaining the solar parallax
Halley was the first to explain how it was to be accomplished. This he
did in a paper read before the Royal Society in 1716 wherein he urged
young astronomers who should live to observe the phenomenon to  apply
themselves dilligently with all their might to the making of this
observation  so that  it may redound to their immortal fame and glory .
He assumed the Sun+s parralax to be 12½ seconds and the difference
between the parallaxes of the Sun and Venus to be 31 seconds. It is
impossible to go into the details of his prediction but it must be
mentioned that he was careful to explain that the motions of the Nodes
of Venus had not yet been discovered and could only be determined by
such conjunctions of sun and planet. If the Nodes remained in the same
place and if the plane of Venus+s orbit were immoveable in the sphere of
the fixed stars the planet would pass four minutes of arc below the
Sun+s centre.
	He laid great stress on observtions being made either at Madras or
Bencoolen on the western coast of Sumatra and also at Fort Nelson on
Hudson+s Bay in North America. At the first names places the whole of
the transit would be visible while at Fort Nelson woudl enter on the
Sun+s disc just before sunset and leave it immeidately after sunrise.
Moreover the duration of the transit would be 15 minutes 10 seconds
longer at Fort Nelson that in the East Indies.
	As the date of the transit approached astronomers of all nations got
busy. The Royal Society raised money, including £800 from the Crown, to
send Maskelyne to St. Helena and Masonand Dixon to Bencoolen. Maskelyne
was unfortunate as bad weather prevented his observing the Transit.
Mason and Dixon embarked on H.M.S. Seahorse but a French frigate of
superior force attacked their vessel and killed 11 and wounded 38 of the
crew. The Frenchmen sheered off when another English vessel hove in
sight but the Seahorse had to put back to repair damages. Our
astronomers had not bargained for fighting and they wrote to the Royal
Society declining to proceed to Bencoolen but offering to go to
Scanderoon. The reply was a letter which (let us hope) has never been
equalled by that illustrious body before or since. It threatened
inflexible resentment and prosecution with the utmost severity of the
law. It prophesied an indelible scandal upon their characters and utter
ruin, and concluded with an express command to go on board the Seahorse
and enter upon the voyage  be the event as it may fall out .
	Fear of the Royal Society proved greater than fear of the French and
the voyage began but they soon found that they could not hope to reach
Bencoolen in the time so by the advice and consent of the captain of
their frigate they made for Cape Town, then of course a Dutch colony. As
good patriots they could not admit to foreigners that an English ship
had had a bad time from a Frenchman so with economy of truth they
informed the Dutch Commander that  diverse disasters  in the Channel had
unduly delayed them.
	They asked to be allowed to spend some time here, that a site suitable
for an observatory should be granted and that materials for the erection
of the observatory should be supplied. All of which requests were duly
complied with and when they left our Astronomers wrote conveying their
grateful appreciation of the assistance rendered to them. The site of
the observtory was between John and Hope Streets behind St. Mary+s
Cathedral. (I may mention here that thanks mainly to Mr. Wells, Capt.
Cook+s chief astronomer, and Sir Thomas Maclear the site of the taking
of every astronomical observation made at the Cape can be ascertained
with geodetic precision.) Its latitude was determined as South 33° 44'
42'' and its longitude East 1hr.13m.35s.
	Mason and Dixon arrived in Table Bay on April 27th, took the
instruments on shore on May 2nd and set the clock going on May 4th. The
body of the observatory was circular with a radius of 6½ feet and the
height of the wall was 5½ feet. The roof which was made of board was
conical in shape and was moveable. The opening was 3 feet broad and the
roof was easily turned to any part of the heavens. The clock was fixed
against two timbers of 10 x 8 inches section sunk 4 feet into the ground
and joined by rods 1½ inch in diameter. The pendulum of the clock was
not altered in length. The other instruments were (1) a quadrant of one
foot radius made by Bird and the property of the Earl of Macclesfield
and (2) two reflecting telescopes, each 2 feet local length and
magnifying 120 times, mady by Short. One at least of the telescopes had
a micrometer because we find a small table  for the adjustment of the
nonus of the micrometer . On May 4th they fixed the quadrant
satisfactorily and found by meridian observations of Procyon on the 4th,
5th and 6th that the clock has a losing rate of 2m 17s, 2m 18s, and 2 m
16½s. The report says that on this date  the observatory being now
finished I put the clock into it, wound up the pendulum and set it to
nearly sidereal time .
	On the great day the sun ascended in the thick haze and immediately
entered a dark cloud but in 20 minutes they obtained their first sight
of Venus which of course was on the Sun+s disc. Then it became first
hazy and then cloudy but at 1h 18m 7s they obtained a measurement of the
distance between the Sun+s farthest limb and Venus+s southern limb. They
made 5 more similar measurements and also determined the apparent
diameters of the Sun and Venus. Very soon after the transit finished the
sky again became cloudy and remained so until night. Mason reports  When
I saw the planet first its periphery and that of the Sun+s were in a
great tremour, but this vanished as the Sun rose and became well
defined. Four minutes before the internal contact the Sun+s disc was
entirely hidden by cloud for about on minute .
	During the remainder of their stay at the Cape our astronomers made
many observations of stars at equal altitudes to obtain meridian
passages, measured zenith distances of various stars for determining the
latitude, and observed immersions and emersions of Jupiter+s satellites
for determining the longitude.
	On September 28 they packed up their instruments and the next day put
them on board the Mercury on which on October 3rd they sailed for St.
Helena where they joined Maskelyne. There the clock was set going again
(the length of the pendulum still not having been altered) and from Oct.
31st 1761 to Jan. 22nd 1762 they made observations for determining the
rate of the clock.
	They left soon afterwards and reached England in safety.