limits have to be applied to the number of observed turning points.
Depending on the precision desired, four to ten observations will
usually suffice for most engineering requirements. Within this range
it is hoped that some high and some low readings will manifest
themselves. As long as the amplitude of the secondary quasi-
harmonic (difference between highest and lowest extremal oscil
lations) is of the order of 30 seconds of arc, for example (as is the
case for GAKi No. 3139), the azimuth determination can be ex
pected to be satisfactory, certainly to the 20 seconds of arc claimed
by the manufacturers.
The quasi-harmonic phenomenon, even although not explainable
in terms of the theory of the gyroscope developed thus far, can be
detected easily by the layman. The amplitude range of the quasi-
harmonic oscillation should be found for every instrument, there
fore, and used as the basis for formulating the procedure to be used
by the technician operator. This range is not a constant in the com
monly accepted meaning of the word but is detectable only as a
trend which is of value to the observer and helps him to decide
whether, or not, a set of observations is likely to be satisfactory.
With the transit method the foregoing pattern is seen as a systematic
trend in dt, the difference between consecutive transit times.
Here, however, the secondary amplitude appears to be smaller
than that encountered with the reversal method. It has not been
possible to study the effect which a change of latitude would have on
these secondary oscillation patterns.
Throughout the tests the manufacturers' handbook [1] was used
as an operations guide, although several changes in recommended
procedure were found to be not only advantageous but also nec
essary. It was found, for example, that because of the particular
lanyard clamp system1) used on the T2 gyro-attachment which
the authors had at their disposal, it could not be mounted on the
theodolite without disturbing the pre-orientation and levelling
this irrespective of how much care was exercised. It therefore
became necessary to mount the gyro-attachment and observation
tube, and to connect the converter cable prior to any levelling of the
theodolite.
Considerations governing the choice of observation method as
outlined in the handbook were subjected to a number of tests
insofar as the tracking procedure was concerned. Criticism of the
tiring effects of this method is perhaps valid, but the rejection of
the technique when the theodolite does not possess a tangent
screw with a range of at least 8° and with a knob at each end is
questionable, if not unjustified. As mentioned previously, the
modified tangent screw on the theodolite covered a range of ap-
274
1) The authors understand that the T2 theodolite is now fitted with a
bridge similar to that used on the TiA and T16 theodolites, to take the
GAKi gyro-attachment.
