70 HYDROGRAPHICAL SURVEYING [cHAr. iii. 



and we want our scale to be near the truth from the first, this 

 method is valuable. 



The only drawback to it is the effect of local attraction on 

 the pendulum, or, in other words, on the mercury in the 

 artificial horizon. With high land behind a station and a 

 deep sea in front it may result that there will be considerable 

 error ; and the difference in the distance between the terminal 

 points of the survey, if it covers much ground, as deduced 

 from such a base, and as determined from the observations at 

 either end, may be much more than if starting from a measured 

 base. 



In order to minimise these effects on the observed diff. lat. 

 or diff. long., the conditions at both base stations should be 

 similar as far as possible. 



The base stations should be as far apart as possible, in order 

 to minimise the effect of any errors in the astromonical observa- 

 tions. 



The observation spots would not necessarily be actually at 

 the base stations, which latter would probably be situated on 

 elevated summits at some little distance, in order to command 

 distant views. 



In such cases, each observation spot would be connected with 

 its corresponding base station by a subsidiary triangulation, 

 a short base being measured for the purpose. 



The sliip, at anchor off the observation spot, frequently 

 affords a convenient means of effecting the connection by a 

 mast-head angle base and simultaneous angles. 



If possible, in the case of a base by diff, lat., the observation 

 spots should be east or west of the elevated base stations ; and 

 in the case of a base by diff. long, they should be north or south 

 of the base stations. This reduces the amount of the correc- 

 tion to be applied. 



BASE BY IVIAST-HEAD ANGLE. 



This consists in measuring, with a micrometer or sextant, 

 the angle between the mast-head of the ship and the hammock 

 netting, or some other fixed line on the ship's side ; not the 

 water-line, as that varies. 



