276 HYDROGRAPHICAL SURVEYING [chap. xii. 



If more than one officer is to observe, a screen of some kind 

 should be put up north and soutli between the observers to 

 keep the lights out of one another's eyes. 



In the tropics and some other localities mosquitoes must 

 not be forgotten. In places where these plagues abound, it 

 is preferable to court the wind instead of shutting it out, in 

 order to free ourselves if possible of them. Sand-flies are 

 perhaps worse, as nothing will get rid of them, and many an 

 otherwise favourable opportunity of getting stars has been 

 spoilt by these wretched little insects. 



The spot for the artificial horizon being settled on, and the 

 direction of the meridian taken with a compass, it is a good 

 plan to dig holes, if the nature of the soil permits, in which 

 to place the lantern used for reading off when not required, so 

 as to avoid unnecessary glare.* The best place for these will 

 be on the left side of, and a little behind, the observer's seat, 

 and two will be wanted for each observer, one for the north 

 stars' position, the other when facing south. If the ground 

 will not admit of digging holes, buckets will answer the same 

 purpose well enough, but not so well. 



If special lanterns can be got, these precautions will not 

 be necessary ; but we are assuming observations with ordinary 

 ships' lanterns. 



All these kinds of preparations should be made before sun- 

 set, if possible ; confusion will be sure to occur if things are 

 delayed till after dark. 



For observations with a sextant stand, a small stool is 

 wanted, as described under " Sextant Stand," and another for 

 the observer's seat. It much facilitates good observations for 

 the observer to be comfortable, especially when he is about to 

 observe for several hours consecutively. 

 Star Map. A good star map is very useful to assist in recognising the 



objects chosen. 



Error of It will have been necessary to obtain the error of the chrono- 



Chrono- meter on the day of our star observations (by single altitude 



required, is sufficient), unless we have recently obtained error at the 



same place, and have confidence that the chronometers are 



going sufficiently well to give us the true time of place to, say, 



two seconds. 



* Some officers now fit their sextants with small electric lights as a luxury. 



