THE ROYAL SOCIETY, AND AT THE ROYAL OBSERVATORY, GREENWICH. 607 
The subjects of this paper are the determination of mean numerical values, and 
the establishment of the laws of periodic variation from the long series of observa- 
tions which were taken under the direction of this Society, combined with that still 
being made at Greenwich. I have not attempted to deduce any rules for non-periodic 
variations. 
It is most fortunate that through all reports this series of observations continued 
unbroken for so many years, and that it did not cease till that at Greenwich had 
been in operation for two or three years. 
The number of observations treated of in this paper exceeds 200,000 spread nearly 
equally over seventy-nine years, and the results generally are important additions to 
science. I consider the determination of the mean temperature at Greenwich as a 
real addition ; it is probably the best determination of this element of any spot on the 
globe, and it will help to an accurate knowledge of the mean temperature of its sur- 
face. 
I may however here remark, that none of the mean results in this paper could have 
been calculated if observations at equal intervals had not been taken throughout the 
twenty-four hours and continued for a few years. The observations at Greenwich 
have supplied this want. It was upon them, taken for five years, I based my deter- 
minations of curve of hourly mean temperature, and by this means have made all the 
observations available. 
As it is difficult to have instruments read more frequently than twice or thrice in a 
day, yet, to make these available, it is necessary that the laws of diurnal changes of 
temperature be known ; and where such is not the case, an effort should be made in all 
countries to have a series of hourly or bi-hourly observations made for a few years, 
so as to be able to deduce useful results. Such series of frequent observations need 
not be made near each other, as it is found that the observations made over a con- 
siderable extent of country are subject to the same general laws. 
