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XVI. On the Reduction of the Thermometrical Observations made at the Apartments of 
the Royal Society , from the years 1774 to 1781, and from the years 1787 to 1843. 
By James Glaisher, Esq., F.R.S., of the Royal Observatory, Greenwich. 
Communicated by John Lee, Esq., LL.D., F.R.S. 8fc. 
Received February 21, — Read May 3, 1849. 
The meteorological observations which have been made at the Apartments of this 
Society, extend over so long a period of time, that if the instruments used have been 
good, and the observations have been faithfully recorded, the results which can be 
deduced from them must be of great value; on the other hand, if either of these 
essentials has been neglected, any results from them would be valueless. 
To the present time, so much uncertainty seems to have rested upon these observa- 
tions, and so much suspicion upon their accuracy, that they have been little used, 
and generally when reference has been made to them, it has been accompanied with 
the remark that the results were not satisfactory, and till recently such was the 
opinion which I entertained myself. 
In the year 1848 I had the honour of presenting to this Society the determination 
of the diurnal variations of the different meteorological elements, and the corrections 
to be applied to monthly mean values of observations taken at any time of the day, 
to deduce from them the true values for the month. 
The accordance which I had found in the diurnal variations year by year, led me to 
suspect that the corrections would apply to a great number of years. To determine 
this I had recourse to the observations of this Society. Throughout this series two 
thermometers have been used in the ordinary daily observations ; each was placed 
2 or 3 inches from a wall, one faeing E.N.E. and tlie other W.S.W. As the sun 
shines on the eastern part of the building in the morning, the thermometer to the 
westward was made use of for the morning observation during that season of the 
year when the sun rose high enough to affect the other : for all other observations, 
that to the eastward was employed. Of these instruments two observations have 
been taken daily, the one before and the other after noon ; the actual times, how- 
ever, have been different at different epochs, and at times different in the same month ; 
these circumstances were favourable for my purpose, though undoubtedly they have 
been highly prejudicial to the character of the journal in consequence of the diurnal 
variations being then unknown, and the mean monthly values as printed differing 
from the true values for the month by different quantities. 
At every variation of the times of observation, and at different epochs with the 
