STANDARD - Survey Instrument Antique Center!

138
Reversion Level.
Applicable to any of our Engineers' Wye Levels*
The spirit level used in this feature differs from the ordinary one in that it is
ground to the true shape of a barrel so that the tangents to the level bubble curves, at the
zero points of the scales, are parallel and diametrically opposite. By the use ot: this reversion
level, attachable to any of our Engineers' Wye Levels, in place of the single reading
level, it is possible to do good leveling, though the adjustments of the spirit level
and cross wires are entirely deranged and the collars worn (see remark below), by first
making the level bubble central and taking a reading, then by revolving the telescope
180 in its wyes, which point is indicated by an adjustable stop,* making the bubble
again central and taking- another reading. The arithmetical mean is the correct result.
This device will, in an emergency, be appreciated when it is known that by the use
of the method above the work will average a^ good as that done with an ordinary good
wye level, in adjustment. The adjustment of an instrument provided with a reversion
level is made in precisely the same manner as if the spirit level was of the single reading
kind, since the adjustment of the level when it is reversed will take care of itself.
The reversion level is guarded by a revolvable outnr tube (Patented, not shown in
cut) leaving a space of air, as a non-conductor of heat between it and the ordinary level
mounting tube. This exterior tube serves both as a protection against breakage and
sudden changes of temperature, and, as its inner surface is painted white, it also acts
as a reflector which facilitates the reading of the bubble.
IJemark: The inequality of worn collars cannot be eliminated in a strict sense
by using the reversion level, yet for ordinary good work it may be said to be. Nor can
the test for the equality of the collars be directly tested in this way but should be done
as in the case of the ordinary wye level ; viz , by the two-peg method described under
the adjustment of the Dumpy Level (pages 63 and 64 of our handbook). The following
modification is to be noted: After the line of collimation has been adjusted for distant
objects by rotating the telescope in its wyes and the spirit level has been adjusted
by reversing end for end and adjusted laterally, (the telescope having the sunshade attached,
as it serves to balance the telescope when the object slide is drawn in), the instrument
is set up close to the near target, and a reading is taken with the level tube
in the direct position. In order to eliminate the error of collimation for nearer objects,
should any exist, another reading is taken with the telescope rotated 180 in the wyes,
and the mean taken as the true reading. If, now, the horizontal wire also bisects the
distant target and the bubble remains central in each position of the telescope, the
collars are of equal diameter. Should the latter not be the case, the error may be corrected
thus : Bisect the distant target with the telescope in its direct position, and adjust
the level till the bubble is central. Rotate the telescope 180 in its wyes, indicated by
the stop, and note the number of divisions through which the bubble moves in order
that the distant target remains bisected, so that a correction can be made when most
precise work is required.
It is assumed that in making this test the temperature of the two collars has been
alike and that the telescope has been in proper balance by being focussed for a distance
of about 300 feet with sunshade attached. A scratch on the telescope or object slide
indicates the focus which the maker used in equalizing the collars. An apparent error
found as above may be due to a change in the shape of the level tube which may occur
in time (for which the maker, of course, cannot be held responsible), as well as to a
worn condition of collars, or these causes combined.
In order to trace the error to its source the only sure test is made with a striding
level. (See Engineer's Precise Level.)
Price, as above, if ordered with our Wye Level in place of the single reading
kind .'.../ $20.00.
Y It is an extremely difficult matter to glint 5 a level of this kind so that the bubble will remain central at
all positions during this rotation through 180. The stop just mentioned is so adjusted, however, by the maker,
that when the level has been turned exactly 180 it gives a correct reading.