230

THE NEW ZEALAND GAZETTE.

MEMORANDUM

On Determinations, by Galvanic Signals, of Differences of Longitude between Burnham Observatory and
other Stations in New Zealand, 1874-75. Also, a Note on Absolute Longitude and Latitude.

1. The stations included in the operations about to be described were,—
   The transit-pier at the transit of Venus Observatory, Burnham
   " " " " " Queenstown.
   " " " " " Government Observatory, Wellington.
   " " " " " Mr. Heale's temporary Observatory, Auckland.
   I will designate these four points by the letters B, Q, W, A, respectively.

2. The Observers who took part in the work were Lieutenants Crawford, R.N., and Darwin, R.E.;
   Mr. T. Heale, Inspector of Surveys; Major Palmer, R.E.; and Professor C. H. F. Peters, Chief of the
   American Transit of Venus party at Queenstown. These observers will severally be distinguished by
   the letters C, D, H, P, PE.

3. The method pursued was as follows:—Reciprocal galvanic signals were exchanged between two
   stations (Burnham being one of them) whose difference of longitude was required, on nights when
   local time was accurately determined at both by transits of Greenwich clock stars. A sidereal clock or
   chronometer was used at one station, a mean solar chronometer—compared with the sidereal standard
   before and after signalling *-at the other. At the sending station an ordinary key was used for
   signalling, the operator tapping his key precisely at every tenth second of his clock or chronometer
   with an additional tap at the fifth second of each minute as a means of identification. This was
   continued for seven minutes. At the receiving station the signals were sounded on a Morse instrument,
   and the Observer noted† and wrote down the instant of reception of each signal, to the nearest tenth
   of a second by his clock or chronometer. When one set had thus been sent, the direction of the signals
   was changed; the station which had at first received now sent. Each station of a pair thus sent and
   received a set of signals on each night of observation. For the determination of difference of longitude
   from any set of signals, the mean of all the times of transmission from the sending station (omitting
   the fifth-second signals), reduced to true sidereal time at place, was used, for comparison with the mean
   of all the times of reception of the same signals at the receiving station, reduced to true sidereal time
   there. The difference of these sidereal times gave one value for the difference of longitude. The
   signals were exchanged when practicable at about the middle time of the star observations, so that the
   times of transmission might coincide pretty closely with that for which the mean clock error was found
   by observation, thus avoiding the necessity of trusting the clock-rate for more than a very short
   interval.

4. The determinations of local sidereal time depend almost entirely‡ on the right ascensions of
   Greenwich clock-stars as derived from Greenwich Catalogues, and on the places of southern circum-
   polar stars furnished in Stone's and Ellery's Catalogues. Strict methods of reduction were adhered to
   in every case. I myself drew up the instructions and tables for use at W and A.

5. The sources of constant errors in the determinations of difference of longitude, apart from
   accidental errors of observation, are the following:—
   (1.) Wave and armature time on the telegraph line; which, however, may be considered as
   eliminated by sending the signals both ways.
   (2.) The personal equations of the observers in observing transit of stars; unknown.
   (3.) The personal equations in tapping the key in coincidence with the clock beats; unknown.
   (4.) The personal equations in receiving signals by ear; unknown.
   (5.) The different qualities of the transit instruments at the several stations.
   (6.) In the case of Q, the difference in the mode of recording transits and time-signals from
   that practised at B, Q having used a chonograph for both these purposes.

6. Signals were exchanged on four nights between B and Q; on four nights between B and W;
   and on three nights between B and A. The results, subject to small errors of the kinds enumerated
   above, are as follows:—

7. Queenstown.

Date. Operators. Resulting Diff. of Longitude. Time, determination by.

Nov. 25 PE at Q sends to P at B ... Q west of B M. S. 14 32.25 } D at B; PE at Q
P at B sends to PE at Q ... " 14 32.25 }
Nov. 27 PE at Q sends to P at B ... " 14 32.63 } D at B; PE at Q
P at B sends to PE at Q ... " 14 32.64 }
Nov. 30 PE at Q sends to P at B ... " 14 32.81 } P at B; PE at Q
P at B sends to PE at Q ... " 14 32.85 }
Dec. 2 PE at Q sends to P at B ... " 14 32.97 } P at B; PE at Q
P at B sends to PE at Q ... " 14 32.94 }

Mean difference of longitude Q west of B ... 14 32.67
The second result on 27th November is the mean of two sets, in which the decimals of seconds
were 61s. and 67s.

• Except at W, where the mean solar chronometer was itself made the standard.
  † Except at Q, where the signals were recorded automatically on a chonograph.
  ‡ Q only used a few stars not in the list of Greenwich clock-stars, and did not directly use the Greenwich Catalogues.
  But the right ascensions used were in every case sensibly the same as those derivable from the Greenwich Catalogues.