The Southwestern Historical Quarterly, Volume 29, July 1925 - April, 1926 Page: 84
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Southwestern Historical Quarterly
Medina, a noted engineer and scholar of Spain. Thus a river is
the first monument to the engineering profession in Texas. Medina
was very much on his mind for in his diary, Palm Sunday, April
3rd, 1689, eight days previous to the discovery of the Medina, we
read the following: ". .. we observed the altitude of the sun
with an astrolabe, though a. defective one, and found our latitude
to be 26 31'. I must call attention to the fact that the tables on
which this observation was based were made before the so-called
Gregorian correction. This correction was made in the year 1582,
in which the equinox was on the tenth of March. Following the
Ephemerides of the Roman Andrea Argoli, which places the equi-
nox this year (1582) on the 20th of March, we found by these
tables that today, April 3rd, corresponds to the 24th of March of
this year 1689, which is the first since the bissextile. These tables,
the author says, he took from the Arie de Navegar by Maestro
Medina. It has been necessary to state these facts in explanation
in case it should appear that a mistake has been made because of
our lack of modern tables."
That De Leon's observations are in error as much as two minutes
has been suggested by Elizabeth West, who translated his journal.
This error came more from the inherent imperfections of the
astrolabe than from Maestro Medina's tables, as feared by De Leon.
Moreover, the instrument was admittedly defective. His entry of
April 20th recites, "Though the astrolabe was broken, we righted
it that day as best we could and made an observation of the sun,
and found ourselves in latitude 28 41' north."
A map of De Leon's route of 1689 exists in the archives at
Seville, Spain. It is reproduced in Volume VIII of this QUAR-
TERITY (preceding page 199). The map is a traverse with num-
bered courses. On the margin is a table giving the bearings and
estimated distances in leagues. De Leon's method was the mar-
iner's "dead reckoning" applied to a land journey.
The trained explorers reported places suitable for settlement,
such as the site of San Antonio, Texas, found by Teran on June
13th, 1691. If the recommendations of such leaders were followed
by establishing towns, the King's military engineers fortified them.
His surveyors laid out the streets, parcelled the land and measured
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Texas State Historical Association. The Southwestern Historical Quarterly, Volume 29, July 1925 - April, 1926, periodical, 1926; Austin, Texas. (texashistory.unt.edu/ark:/67531/metapth117141/m1/98/: accessed January 20, 2018), University of North Texas Libraries, The Portal to Texas History, texashistory.unt.edu; crediting Texas State Historical Association.