Klotz. — Transpacific Longitudes. 5S 



Bamfield, on the west shore of Vancouver Island, is the 

 eastern end of the Pacific cable, and was not occupied as an 

 astronomic station, but simply as an exchange station — that is, 

 for the comparison of the Fanning and Vancouver chronometers,, 

 to be described more fully later. 



Longitude work consists in simply determining the accurate 

 sidereal time for each of two places, the longitude of one of them 

 being known, at an absolute instant, and then comparing such 

 times : the difference between them will be the difference in 

 longitude. The operation may be briefly stated : Each observer 

 determines the error of his sidereal chronometer at a particular 

 instant ; then by means of the telegraph line or cable the two 

 chronometers are compared, to be explained later ; this com- 

 parison may be likened to an instantaneous photograph of both 

 chronometers. Applying the respective chronometer corrections 

 for the instant of comparison to the times thus shown by the 

 two chronometers, we obtain the absolute local sidereal time 

 for each place for the same instant ; and, as before, the difference 

 between these times is the difference of longitude. 



Now, suppose we have a transit instrument with a single 

 vertical thread, and that thread situate in the axis of collimation ; 

 furthermore, the axis of the telescope horizontal, no inequality 

 nor ellipticity of pivots, and the pointing of the telescope truly 

 in the meridian ; then, if we record the transit of a star across 

 the thread, and the time noted is free from personal equation, 

 we obtain immediately the clock-corrections by comparing the 

 observed time with the right ascension of the star for that time 

 and day. The many conditions imposed in the last sentence 

 show^ the many sources of error, the effect of which must be 

 evaluated ere we obtain the desired quantity — the clock-correction; 

 in other words, the true local sidereal time at a given instant. 



We must therefore devise means for determining the instru- 

 mental errors, some of which are practically constant — inequality 

 and ellipticity of pivots ; while the others — level, azimuth, and 

 collimation — are more or less variable from day to day. Careful 

 readings, at the beginning and end of a season, of the former 

 will evaluate them. For the latter we will speak of the level- 

 corrections first. This quantity is determined directly by means 

 of the striding-level placed upon the axis of the instrument. 

 Readings should be taken as frequently as the intervals between 

 stars admit. With sensitive levels, reading about a second of 

 arc for divisions, great care must be exercised in allowing the 

 level to come to rest. My own practice is not to take a reading 

 until fully a minute has elapsed after placing the level, and as 

 a light is necessary for reading at night, the reading should be 

 taken quickly, for even a short exposure of the level to light 



