NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
DIP
From: Willem Piccer
Date: 2005 Dec 3, 17:27 +0100
From: Willem Piccer
Date: 2005 Dec 3, 17:27 +0100
In an old study-book I found this method of finding the
dip.
I sent an attachment to Robert Gainer at navl@fisherietrust.org
Prism for the measurement of the dip
On some sextants a device can be placed, which gives an
opportunity to measure the angle that the directions towards two points on the
horizon diametrical opposite to each other make
Supposing that in both directions the dip is the same, one
finds ( after apllication of the I.C.) double the value of the dip.
For this purpose a rectangular prism A is put in front of
the non-silvered part of the horizon mirror.
This prism has a foot with a square pin which is slipped
into a box which is placed in the frame of the sextant
The prism can easily be placed or removed.
To measure the dip, a prism B is screwed on to the
telescope or the eye-tube, by which it is possible to see through the telescope
the pictures which are formed in the silvered part of the horizon mirror and in
the prism A
To be able to keep the eye during the measurement on some
distance from he plane of the sextant ( which is desirable, because otherwise
the head of the observer will intersept too much light) sometimes a tube is
mounted on the prism B
One keeps the instrument vertical such that one has the
telescope left and the horizonmirror right, meanwhile looking sideways through
prism B
The instrument is kept in such a position, that the
picture of an on the left side lying part of the horizon through reflection is
seen in prism A ( in the picture K2 gives the direction of the for the observer
on the left side lying horizon)
By moving the alidade one brings the double reflected
image of an on the right side lying part of the horizon in line with the first
mentioned image ( K1 shows the direction of the horizon on the right side; K1
does not go through the prism you will see it next to it!)
Through the rectangular prism A the beam K2 will be
reflected parallel to it self, so that the reading of the instrument, after I.C.
has been applied, gives the angle between K1 and K2. This angle is double the
dip.
In general one finds a negative reading.
I have never seen this and hope the translation is good
enough to give you a good understanding how it works
Have fun
Willem Piccer
.
