February 27, 1903.] 



SCIENCE. 



3-29 



to the motion of the base of the pier. In 

 order to iinderstand how the temperature 

 acts on meridian instruments, we need 

 some physical constants. If we assume 

 the conductivity of iron as one, or unity, 

 mercurj' is 1/10, stone, brick and wood 

 1/130 to 1/180. It will be readily under- 

 stood that the iron outside of the pier will 

 act quickly as a thermometer, the iron in- 

 side the pier will act more slowly, and the 

 supporting piers will act very slowly in 

 taking the temperature of the external air. 



The piers acting as a thermometer may 

 lag one month or more, and this is the ex- 

 planation of the phenomenon observed at 

 Edinburgh. Hence we conclude that vari- 

 ation of level and azimuth during a night 

 of observation is almost entirely due to 

 the effect of temperature on the metal 

 parts of the instrument. The covering of 

 piers with cloth and wood is of no use. 



IMany instruments in use change their 

 level and azimuth by jumps, and not in 

 an.y regular manner. If the expansion of 

 iron is taken as unity, brass is 2, sandstone 

 and granite 0.8 to 0.9, and bricks from 0.3 

 to 0.5. It is readily seen that the differ- 

 ence of expansion between brick and iron 

 is so great that the instrument will always 

 be loose on the piers. Hence it is free to 

 jump in both level and azimuth. 



In the Pistor & Martin's meridian circle 

 the brass ej-linder for holding the Y should 

 be replaced with iron. 



The modem Repsold is defective in its 

 mechanical construction, for the reason 

 that the T-piece and the counterpoise 

 weight are all supported on one frame, and 

 when the instrument is reversed it is liable 

 to be disturbed in level and azimuth. The 

 Dearborn Observatory old pattern Repsold 

 meridian circle is mounted on sandstone 

 piers, and the lugs for holding the T-pieees 

 are set in with lead. The in.strument is 

 absolutely stable in level and in azimuth. 



The computed monthly level for two years, 

 when corrected for temperature and the 

 motion of the pier, agrees with the ob- 

 served level within a fraction of a second 

 of are. 



The Probable Value of the Constant of 



Aberration: S. C. Chandler. 



The number of detei-minations of this 

 constant is now so considerable that even 

 wide differences of judgment as to the 

 weights to be assigned them can have but 

 little influence on the mean result. Forty- 

 three determinations are combined with the 

 following weights: 



Talcott's method 20.523 Weight, 151 



ileridian declinations 514 22 



Prime vertical transits 525 24 



Right ascensions 53 6 



Prismatic apparatus 48 5 



Mean 20.521 20S 



The Constant of Aberration from Observa- 

 tions with the Zenith Telescope, 1901- 

 1902: C. L. DooLiTTLE. 

 A preliminary reduction of the series of 

 zenith telescope observations covering the 

 period from October 1, 1901, to October 

 1, 1902, gives for this con.stant the value 

 20".510. 



This is preliminary in the sense that 

 some of the woi'k of reduction has not been 

 fully verified and that it is proposed to 

 include in deriving the final result some 

 additional data, viz., about four hundred 

 observations between October 1, 1902, and 

 January 1, 1903. 



The values derived from the different 

 series of observations at the Sayre and 

 Flower Observatories are as follows: 



(1) 1889-1890 20.448 ± 014 Weight! 



(2) 1892-1893 20.551 ± 009 1 



(3) 1894-1895 20.537 ± 014 1 



(4) 1800-1898 20.5S0±008 * 



(5) 1898-1899 20.540 ± 010 1 

 (G) 1900-1901 20.501 ± 008 1 

 (71 1901-1902 20.510 1 



\\(i.;;liled mean 20".530 



rnufif..|ited mean 20 ..532 



