390 



Mr. E. J. Stone on a Decennial [Mar. 23, 



mometers which have been employed in this series of observations. I 

 have then deduced the rate of change of these thermometers, from a com- 

 parison of the index-errors thus found and those given originally or ob- 

 tained in 1852 by Sir Thomas Maclear, when he compared the principal 

 thermometers at the Observatory with the readings of a standard " Reg- 

 nault" which had been sent out to the Observatory for that purpose by 

 you. These indications of change have been carefully checked by all the 

 comparisons made, at different times and for different purposes, of these 

 thermometers inter se and with others which still remain at the Observa- 

 tory. From the agreement of the different results thus checked, I have 

 no doubt upon my own mind of the systematic character and sensible 

 amount of the increase of readings of thermometers with age thus indi- 

 cated. In some cases the change appears to amount to as much as 

 o, 05 F. per annum. From these results I have deduced the index-errors 

 of the different thermometers for the different periods, and applied these 

 corrections throughout. I have also corrected the mean results of the five 

 observations made daily since 1847 in order to deduce the true daily mean. 



The results thus reduced on a general system, and extending over thirty 

 years, appeared likely to afford information respecting any connexion which 

 might exist between the mean temperature and the frequency of solar 

 spots. I have therefore constructed the curves of variation of mean annual 

 temperature, and the inverse curve of solar-spot frequency for comparison. 

 The latter curve has been founded upon Wolf's observations. 



The observations of temperature from 1841 to 1851 inclusive were made 

 in the original Meteorological Observatory, which was burnt down in 

 1852, March 11. 



The observations from 1852, April 24, to 1858, August 31, were made in 

 a wooden shed erected for the purpose on the site of the old Observatory. 



The observations from 1858, August 3 1st, to the present time have 

 been made in the crib before the south-west window of the Transit-Circle 

 Room. 



These changes are so far unfortunate that there is clearly a change of 

 mean temperature arising from the different circumstances of exposure. 

 I have therefore referred each set of observations to the mean temperature 

 deduced from all the observations made under the same circumstances of 

 exposure. The deviations of the mean temperature for each year from the 

 mean of the whole period of similar exposure are then laid down as ordi- 

 nates on the scale of one division of the ruled paper to 0°*05 F. To 

 smooth down the irregularities, I have joined the points thus laid down, and 

 bisected the lines thus joining these points whenever the corresponding 

 mean temperatures were deduced from a full year's observations. In other 

 cases the temperatures corresponding to the deficient months have been 

 supplied from the adjoining years, and the resulting mean temperature 

 allowed less weight. The inverse curve of the frequency of solar spots 

 has been formed by simply subtracting 100 from Wolf's numbers, and 



