AN IMPROVED REFLECTING CIRCLE OR SEMICIRCLE
Year: 1811

page_text: TO ALL TO WHOM THESE PRESENTS SHALL COME, I, Sir HOWARD
DOUGLAS, of High Wycombe, in the County of Bucks, Baronet, send greeting.,
WHEREAS His most Excellent Majesty King George the Third did, by His
Letters Patent under the Great Seal of the United Kingdom of Great Britain
and Ireland, bearing date at Westminster, the Second day of July, in the
fifty-first year of His reign, give and grant unto me, the said Sir Howard
Douglas, my exors, adiňors; and assigns, His special licence, full power, sole
privilege, and authority, that I, the said Sir Howard Douglas, my exors,
adiiors, and assigns, should and lawfully might, during the term of years
therein expressed, make, use, exercise, and vend, within England, Wales, and
the Town of Berwick-upon-Tweed, my Invention of "AN IMPROVED REFLECTING
CIRCLE OR SEMICIRCLE;" in which said Letters Patent there is contained a
proviso, obliging me, the said Sir Howard Douglas, by an instrument. in.
writing under my hand and seal, to, cause a particular description of the
nature. of my said Invention, and in what manner. the same is, to be performed,
to be inrolled in His Majesty's High Court of Chancery within two calendar
months after the date of the said recited Letters Patent, as in and by the same,
relation being thereunto" had, may more fully and at large appear.
NOW KNOW YE, that in compliance with the said proviso, I, the said
Sir Howard Douglas, do hereby declare that my said Invention, and the

page_text: manner in which the same is to be performed,. are described by the Drawings
hereunto annexed, and the following description, that is to say :-
The objects of my Invention of the reflecting circle or semicircle were to
combine the measuring principle with a circular or semicircular protractor in
such manner that in measuring any angle the index or limb of the instrument
shall pass over the whole of the measured angle. By this contrivance any
angle taken in the field may be at once protracted in actual magnitude without
the trouble of reading off. This is particularly useful in military surveying,
where the true situations of objects can at once be determined, and the sketch
corrected at the same time that it is taken. A vernier is applied, to read off
the angle with more accuracy.
reflecting circle or semicircle is
divided into 360° or 180, instead of double the number, as in the repeating
circle, and the same length of arc is obtained to ingrave the divisions upon
which a sextant of a radius equal to the length of the sliding bar would
have.
Thus, the
The description of the instrument is as follows:-To the radius or limb of
a semicircular (or circular) protractor A, B, C, the index glass D, E, is fixed.
The horizon glass F, G, is fixed to a bar H, I, which has a motion on K.
This bar slides upon a pin O, attached to the limb or radius carrying the index
glass; which pin is adjusted so that there may be no apparent index error, and
exactly in the same circle with the point K. The sliding bar will then move
over half the real angle, and the principal limb protract the true one.
In witness whereof, J, the said Sir Howard Douglas, have hereunto set
my hand and seal, the Twenty-ninth day of July, in the year of our
Lord One thousand eight hundred and eleven