rORllECTTOyS HOW OBTAINED. 



During the whole time over which the observations extend, 

 the instruments have occupied the same situation. Up to the 

 end of June, 1849, they were all read daily at two hours, 

 Gottingen mean time, which corresponds with one hour 

 twenty minutes Greenwich mean time. Since the end of June, 

 1849, they have all been read daily at 1 p.m., Greenwich 

 mean time. This difference of twenty minutes in the times of 

 reading has been taken into account in the application of 

 the corrections to the instrumental readings for diurnal range. 



The index correction which has been applied to all the 

 readings of the barometer, to make it read the same as the 

 Royal Society's flint-glass standard, was obtained by the trans- 

 mission of a portable barometer of Troughton's between the 

 Greenwich and Liverpool Observatories. The index correc- 

 tion which has been applied to each of the thermometers, was 

 obtained by comparison with a pair of thermometers, which 

 had been previously compared with the standard thermometer 

 at the Eoyal Observatory. The index correction was found 

 to be so near the same in different parts of the scale of each of 

 the thermometers, that no alteration has been made for error 

 of scale. 



The corrections for diurnal range have been derived in the 

 following manner. During the five years ending December, 

 1845, meteorological observations were taken at the Royal 

 Observatory at Greenwich at every even hour of Gottingen 

 mean time. IVom these observations tables were formed by 

 Mr. Glaisher, shewing the corrections to be applied to the 

 monthly mean of the daily readings of each instrument at any 

 hour, in order to deduce the true mean reading for the month 

 from observations taken at that hour. These tables were 

 published in the Transactions of the Royal Society, part 1, for 

 1848. For the Barometer the corrections for the different 

 months in the year range from + 0*007 in. to — 0*006 in., for 

 the hour at which these observations have been made, and 

 1G8 



