12 



Mr. J. P. Gassiot on Observations 



[June 20, 



This correction is, however, small ; and as the change of mean baro- 

 metric pressure between the different latitudes is small also, the cor- 

 rection will not affect the range, but it will affect the comparability of 

 individual observations, and ought therefore to be determined. This 

 is best done by means of the observations themselves after the tem- 

 perature corrections have been applied. 



Thus we find that, when the ship was stationary at Plymouth, there 

 were considerable fluctuations of the barometer, and from the obser- 

 vations made there, it appears that a rise of half an inch creates a fall 

 in the reading =0*18. Similar fluctuations took place while the ship 

 was stationary at Magellan, and from these we obtain a fall of 0*24 for 

 every rise of half an inch. 



The mean of the two, or a fall of 0"21 in the reading for a rise of 

 half an inch, may safely be adopted. 



No correction has been applied for the hygrometric state of the air. 

 M. Jamin has found that aqueous vapour, at the temperature of 0° 

 Cent., and under the pressure of 0*76 metre, would have for its index of 

 refraction (if it could exist under such a pressure) the value of 1*000261, 

 which is less than that of air, which is 1*000294 ; the superior quantity 

 of aqueous vapour would thus tend to diminish the index of refraction 

 of air at the equator, as compared with that of the same pressure at 

 higher latitudes. 



This action of vapour, which is very small, will be such that without 

 it the residual range (which will be afterwards exhibited) would appear 

 to be somewhat larger than it at present appears ; it cannot therefore 

 account for the residual range, and may be in the meantime neg- 

 lected. 



If we now tabulate from the diagram sent by Captain Mayne, and 

 if, by means of the above-named corrections, we reduce all observations 

 to 60° Pahr. and to 30 inches barometric pressure, we obtain the fol- 

 lowing readings for the various latitudes between 45° JN". and 45° S., 

 the ship being in motion. 



Note. — Since the barometer correction is small, and the range of 

 temperature from the equator to the high latitudes of the voyage not 

 much more than 20° Pahr., it has been thought unnecessary to reduce 

 the barometer readings to 32° Pahr. 



