:jofi 



HYDROGRAPHICAL SURVEYING [chap. xiii. 



Sights 

 missed. 



Mk' mean of the fourth column is 14-5 seconds. We substitute 

 this for the 140 seconds obtained by adding the tAvo times at 

 the middle observation, and, dividing by two, we get the mean 

 middle time by the set as 11 hours 51 minutes 37-22 seconds. 



In cases where the equation of equal altitudes is varying 

 rapidly, we shall not find the middle times of two successive 

 sets agreeing exactly, as they should differ by the amount of 

 the variation of the equation of equal altitudes in the time. 



h April 3nl, 1880, at Nagara Light-house a. 

 Time by Breguet (2086). 



Interval in 

 second-. 



28-0 

 27-6 

 27-2 

 28-0 

 27-2 

 27-4 

 28-2 



28-0 

 28-0 



Time by watch. 



Interval in 

 seconcis. 



h. m. 



3 40 

 39 

 38 

 37 

 36 



04-0 

 36-4 

 08-2 

 41-0 

 13 2 

 45-2 

 17-6 

 49-6 

 22-2 

 55-0 

 27-2 



11 ) 5-0 

 14-45 



Times at middle sight 



h. 111. 8. 



8 05 L'8-8 

 15 37 45-2 



2 )23 43 14-0 

 Mean mid. time by watch 11 51 37 '22 



N.B.— The seconds of the result are obtained by halving the mean of 

 Column 4. 



Theoretically, this is an objection to long sets of observation, 

 but practically the errors of observation will exceed any little 

 discrepancy introduced by assuming the equation to be uni- 

 formly variable during a set of eleven to fifteen observations. 



If a contact is missed in either half -set, it is no use to inter- 

 polate a time. The sight must be missed out of the double 

 set altogether. It will not affect anything but the number of 

 observations in the set, except when it happens to be the first 



