14 



Mr. J. P. Gassiot on Observations [June 20, 



call, and correcting these as before for temperature and atmospheric 

 pressure, we obtain the following results : — 















Station. 









reading re- 



Readings 









Date. 



duced to 



corrected 











60° Fahr. & 



for change 



Name. 









30 inches 



of zero. 



Latitude. 





pressure. 







o 





1864. 







Plymouth . . . 



50 



22 N. 



Sept. 15 ... . 



4-72 



4-72 



Funchal . . . 



32 



38 



Sept. 28 to Oct. 2 . 



4-42 



4-53 



St. Vincent . . . 



16 



54 



Oct. 9 to 13 . . 



4-22 



4-44 



Rio Janeiro . . 



22 



55 S 



J^ov. 3 to 13 . . 



4-29 



4-62 



Monte Video Bay . 



34 



54 



jSTov. 23 to Dec. 3 . 



4-09 



4-53 







Dec. 1866 to Jan. 







Magellan Straits . 



52 



30 



1867 . . . . 



4-16 



4-72 



We have here a result precisely similar to that obtained by consider- 

 ing the observations when the ship was in motion, and we may exhibit 

 the following Table, showing the residual unexplained difference appear- 

 ing to be connected with latitude, as determined by these observations 

 when the ship was at rest. 



Station. 



Residual 

 difference from 

 Plymouth. 



Name. 



Latitude. 



Plymouth 



50° 22' N. 







Funchal 



32 38 



-19 



St. Vincent . . . . 



16 54 



-28 



Pio Janeiro . . . . 



22 55 S. 



-10 



Monte Video . . . . 



34 54 



^19 



Magellan 



52 30 







Thus, whether we take the obervations when the ship was in motion, 

 or those when she was at rest, we find a very perceptible residual 

 difference, the cause of which is unknown, and the tendency of which 

 is to make the readings at the equator about 0'33 revolution lower 

 than those at high latitudes ; a decrease in reading, it may be well 

 to mention, denotes a decrease of refraction, as may be seen by con- 

 sidering the construction of the instrument, and as has been determined 

 by direct experiment. 



J. P. G. 



Clapham Common, May 25, 1867. 



In forwarding the preceding communication to Professor Stokes, 



