316 HYDROGRAPHICAL SURVEYING [chai-. xiii. 



posing the set, combined with the range in the results of the 

 various sets. 



The foregoing tables gi ve empirical values, which experience 

 shows may be used to obtain weights that are at least reason- 

 able. 



The value of each set is found by taking from Tables I. and 

 II. the proper quantities, and multiplying one by the other. 

 The weight to be assigned to the total number of observations 

 by each observer is the sum of the values of each set. 



Error by Equal Altitudes of Two Stars on Opposite 

 Sides of the Meridian, by Captain A. M. Field, R.N. 



The principle of this method depends upon the sidereal time 

 of passing the meridian of a place, by an imaginary star having 

 the mean right ascension of the two stars selected, being 

 compared with the time shown by a sidereal clironometer at 

 that instant ; the difference is its error on sidereal time. A 

 mean solar chronometer can be used equally well. The 

 sidereal time required is the mean of the right ascensions of the 

 two selected stars. The clu"onometer time at that instant 

 (mean or sidereal) is the mean of the times at which the eastern 

 and western stars had equal altitudes, with the " equation of 

 equal altitudes " applied with its proper sign. 



When preparing a list of stars for observing, it will be 

 necessary to first find the R. A. of the meridian for the times 

 between which it is required to carry on the observations. 

 As stars will generally be observed witliin 4^ of the meridian, 

 the limits of R. A. of the stars falling within the required 

 period may be obtained by subtracting 4'* from the first R. A. 

 of the meridian, and adding 4*' to the last R. A. of the meridian 

 as found above. 



To arrange the stars in pairs, it is necessary to select two 

 bright stars of nearly the same declination, not differing much 

 from the latitude, but differing in R. A. by from 4*^ to 8^\ 

 The time at wliich they will be simultaneously of equal altitude 

 will be about the time when the mean of their R. A.'s is on the 

 meridian. 



The time at wliich it will be necessary to begin observing 

 will be governed by this, and the observations of one star 



