I.') 



Sect. I.] 



ASTRONOMY. 



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satellites may also be seen ; and (if the navigator have a 

 teler^cope somewhat better than is usually carried in ships^ 

 and steo "^ily mounted) the passage of Jupiter's satellites, 

 either behind the planet or in front of the planet, may be 

 seen, and the times at which the centres of the satellites 

 just touch the edge of the planet may be observed, Al] 

 these observations will be useful : the observation^^ must 

 be recorded wic^i the same fulness wdiicb. has been men- 

 tioned before. 



4 It may chance that the navigator is in some climates 

 where the air is much more damp, and in others where it 





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is much more dry, than in Europe. It is possible that in 



these places he-may be able to make observations which 



will throw some light upon the influence" of moisture in 



atmosj)heric refraction. It is recommended that repeated 



observations of the altitude of the sun s upper and lower 



limb be taken when the sun is very near the horizon. It 



will be necessary that the time at the ship and the latitude 



be very well known. Tlie thermometer must be read, as 



also some hygrometrical instrument, and the barometer, if 

 there is 



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one on board, during the observations. 



The 



observations of every kind must be recorded witii the 

 utmost fulness. 



5. It IS certain that some of tlie stars of the southern 

 hemiophere are var^-^^le in magnitude ; the most remark- 

 able of these is n Argus. It is desirable that, on farour- 

 abie nights, the magnitude of this star should be ob" '-ved 

 and recorded. The best way of doing it will be, not to 

 state that it looks like a star of the 2nd magnitude, or of 



the 



gnitud 



with that of some of the stars near it. Thus it wilJ be 



