282 HYDROGRAPHICAL SURVEYING [chap. xii. 



7. Add these quantities together, and divide the sum by the 

 number of observations, to get a mean. 



8. Add together log cosine decHnation, log cosine estimated 



latitude, log secant true altitude, and the logarithm of the 



result of No. 7. If we are using versines, a constant log 



9-316400 is also to be added. (This is log cosec r' + 4 



+ 0-002000.) 



9. Look out the sum of these logs as a natural number, which 

 will be the number of seconds of reduction required. 



10. Add this to the mean observed true altitude, which will 

 give the calculated meridian altitude, from which the latitude 

 is obtained in the usual manner. 



An example is given on pp. 283 and 284. 



Pole Star. When the pole star is observed, it must be worked out by the 

 rule given in the "Nautical Almanac," care being taken to 

 take out all the quantities from the tables exactly by inter- 

 polation. 



Moon not The moon is but of little use for observations of any kind. 



^°^' . Its rapid motion necessitates very careful corrections, which 

 take more time than they are worth, and besides, we have 

 nothing to put against it to eliminate errors. 



Results of The separate stars being worked out, we mean the result of 

 each pair, and the mean of these again will give us the mean 

 latitude. 



Although from circumstances many more observations may 

 be got of one star of a pair than of the other, no value can be 

 assigned to one over the other, and the direct mean must be 

 taken ; but in meaning up the results of pairs, less value Avould 

 be given to a pair in which the observations of one star are few 

 in number than to a pair where a proper number of observa- 

 tions of each star has been obtained. The necessity for assign- 

 ing this value is increased where the observations are not only 

 few but indifferent ; but it would be a question whether it 

 would not be better to omit such a pair from the final result 

 altogether, and certainly it would be best to do so, if there are 

 several other good pairs. 



An example of the method of tabulating the different 

 observations and pairs is given on p. 285. 



The sets of stars given as an example were taken under 

 favourable circumstances of sky and weather, and are not 



Pairs. 



