426 THE POPULAR SCIENCE MONTHLY 



one of which culminates on one side of the zenith and the other at 

 nearly the same distance on the opposite side of the zenith, by means 

 of an eye-piece micrometer rather than a graduated circle, and herein 

 lies the chief advantage of the Talcott method over all others, the 

 micrometer being a much more delicate and accurate instrument than 

 the graduated circle. 



The program of work calls for sixteen determinations of the lati- 

 tude each night, which means the observation of sixteen pairs of stars. 

 Particular stars have been chosen in such a way as to give convenient 

 intervals between the culmination times of each and the work con- 

 sumes four hours of time each night. As the stations are all located 

 on the same parallel of latitude the zenith of each observatory will 

 traverse the same path in the sky and the same stars may therefore be 

 observed at each station. Exactly the same program of work, weather 

 and other conditions permitting, is carried out at each station every 

 night of the year. About 12,000 determinations of the latitude are ob- 

 tained each year, the total to the beginning of 1908 being 99,313. The 

 greatest number of observations are obtained at the Italian station, the 

 next greatest at Ukiah, as may be seen from the following table : 



Total of Latitude Observations up to 1908 



Mizusawa 13,561 Cincinnati 12,190 



Tschardjui 14,901 Ukiah 18,676 



Carloforte 25,302 Total 99,313 



Gaithersburg 14,683 



middle is measured by means of the micrometer. The instrument is then 

 reversed about its vertical axis, without disturbing the setting, and the telescope 

 will then point as far south as it did north of the zenith before reversal, or 

 vice versa. The second star will then pass through the field of view as far below 

 or above as the first star was above or below the center, and this distance from 

 the center is again measured by means of the micrometer. The proper combina- 

 tion of the micrometer settings on the two stars gives the actual difference of 

 their zenith-distances, which may be turned into arc measure, provided the value 

 of one revolution of the micrometer-screw be known. The latitude, <j>, of the 

 place of observation is computed by means of the formula, 



<j> = i{8 n + 5 s ) + i(w„ — m s )R + i{l n + l s ) +i(r n — r s ), 



in which the first term of the right-hand member of the equation represents one 

 half the sum of the declinations of the two stars of the pair observed; the 

 second term one half the difference of the zenith-distances of the two stars as 

 measured by means of the micrometer; the third term a small correction for 

 any change in the pointing of the telescope after reversal, detected by means of 

 two very delicate levels attached to the telescope; and the last term a small 

 correction for the difference in the atmospheric refraction affecting the rays ot 

 light coming from the two stars. It might be noticed that if the two stars are 

 at exactly the same zenith-distance, and the instrument is reversed without dis- 

 turbing the pointing, then the second, third and fourth terms each become zero 

 in the equation above, and the latitude is simply the mean of the declinations 

 of the two stars, or the declination of the zenith, as may be seen by referring 

 to Fig. 1. 



