LONGITUDE OF McGILL OBSEKYATORY. 



113 



Date. 



1884. 

 April 21.... 

 May 28.... 

 April 23.... 

 .lune 14 ... . 

 May 22.... 

 May 2.... 

 May 23.... 

 'S\:\y 21.... 

 .Iniic 11.... 

 May 21!.... 



June lit 



]\Iay 19.... 

 .June. 10.... 

 .June IS.... 

 June 17.... 



Correction 

 (L.W.) 



S. 



■010 



•017 

 ■017 

 ■007 

 ■0(17 

 ■010 

 •0(15 

 •(Kil 

 ■000 

 •004 

 ■(104 

 •000 

 ■008 

 ■008 

 •015 



.0010. If there is really the change in the diameter of the pivots which the observations 

 seem to show, the temperature which corresponds to zero correction is VO'. The time 

 observations in this longitude determination were made at temperatures ranging between 

 GO' and 74°, and no pivot correction has been applied. 



The distance between the pivot bearings is 27 inches. There is no clamp, the setting 

 being done by touch. There are two small setting circles at the eye end. 



The reticule is ruled on glass and consists of five groups of five lines each, with the 

 middle group subdivided half-way up the field so as to give a group of thirteen lines. 

 The outside groups are intended only for personal equation work and the central group of 

 thirteen lines for polar star observations. The ec[uatorial distances between the means of 

 each of the three central groups and the mean of all the wires in those groups were 

 determined from fifty-four complete transits of stars between 30' south and 60' north, with 

 the result l'y*10, zero, and 15\T0. An independent determination of the distances between 

 adjacent lines was also made from twenty-seven polar stars, which proved that these also 

 are essentially eciual and have a value 2'-62. As the space between the groups was 

 designed to be twice that between the lines, the distances between the lines as deduced 

 from the group distances, given above, is 2\617, or practically the same as by the direct 

 determination. The reticule was ruled by Prof. Rogers, and is mounted on a horizontal 

 micrometer slide. The value of one revolution of the micrometer screw is 6'3. 



The Sfri'Iing- Level is by Messrs. Fauth & Co., "Washington. It is protected from rapid 

 changes of temperature by an outside glass tube. The scale vakie of this level, as deter- 



' The temperature of the air on April 22nd, May 22nd and 23rd, is uncertain, but was probably within 2° of 



what is given. The greatest deviation of any pair of level readings from the mean of the set was 0'.006, and the 



mean deviation was 0'.0025. 



Sec. III., 1885. 15. 



