DEPARTMENT OP MERIDIAN ASTROMETRY. 155 



The difficulties in mounting and adjustment of the Olcott meridian-circle 

 were rather unusual. The wye-plates are fastened upon large brass rings 

 set into large holes in the pier. The microscopes are supported within sim- 

 ilar holes in the pier and accurately perpendicular to the divided circles. It 

 was necessary that these ten holes should be molded into the pier very exactly 

 and the i8 rings for the support of the telescope and the microscopes had to 

 be fastened into the piers with still greater precision. The work required to 

 fulfill these conditions was quickly accomplished by the members of the staff 

 without other skilled assistance. 



On April 6 the instrument was ready for use in regular observations. 

 Series No. i is of date April 6. Between that date and July 26, according 

 to the latest report, 17,291 observations were made in 136 series, upon 86 

 nights. This was at the weekly rate of 1,080 observations. Taking out 

 15 Sunday nights, there were 10 wholly cloudy nights. This proves to have 

 been more than the average number for the season. At this rate the annual 

 output of observations would be 56,000 per year; but the period over which 

 these observations extend should be the most favorable of the year. The 

 rate of observation is about four times the yearly average rate attained at 

 Albany at periods of greatest activity. The observers have usually worked 

 in two shifts, one for the day and the first half of the evening, and the other 

 for the last half of the night. The necessity of providing the numerous 

 stellar observations required to ascertain certain systematic errors, always 

 peculiar to observers and instruments, has, however, been somewhat neg- 

 lected, and adequate attention to such requirements in the future must cut 

 down materially the present rate of observation. 



So far as can now be judged from the records of measurements upon 

 the Mire and Nadir, the collimation, level, and azimuth of the instrument 

 have remained fairly constant, so that there is very slight ground for ascrib- 

 ing any serious instability in the piers or to suppose that the ''seasoning" 

 of the large piers was insufficient when the observations began. 



The full success of the second expedition rested very largely on the out- 

 come of our efforts to transfer the meridian-circle from its piers in Albany 

 to the duplicate piers in San Luis, without the smallest strain or deforma- 

 tion of its parts ; so that it should be found in its nev/ site, humanly speaking, 

 identical in all respects to that which it was while last in use at Albany. 

 The smallest shock, or accident, or any default in remounting it, might 

 easily defeat this object. It seems worth while to introduce a brief sum- 

 mary of the observations and tests by which we are convinced that our 

 attempts have been successful. 



As stated at the outset, during the period between the first and second 

 expeditions, November 15 to January 20, we had carefully determined the 

 flexure of the instrument. This substantially confirmed previous results. 



