ON THE KEW OBSERVATORY. 355 



In column B the letter P indicates a positive and N a negative charge of 

 the conductor, &c., as ascertained by the use of the Distinguisher. 



In C and D the electrical observations of tension are noted as observed by 

 means of the Volta- and Henley-electrometers. The letter V^ refers to Volta's 

 standard electrometer (whose scale is divided in half-Paris lines as stated), 

 and V'^ to his second, whose scale divisions are each equal to 5 of the V' (as 

 also stated). The value of tension, as observed by means of V®, is set down 

 in terms of V ; the letter H refers to the Henley, one degree of whose scale, 

 in the lower readings, is equal to about 100 divisions of Volta. (It has sel- 

 dom been used for the frequency observations.) 



The columns E, F, G, H have not been used. 



In I are contained from 3 to 7 daily results, carefully obtained between 

 September 10, 1850, and iVIarch 29, 1851, by methods somewhat differing 

 from those which were adopted under instructions to the observer last year. 



" Slips of paper, about 1 8 inches long, were affixed to the observer's clock. 

 The brass time-scale, which is divided in accordance with the rate at which 

 the index travels over the paper, was applied to the slips, and spaces corre- 

 sponding to every 60 seconds marked off on the paper. At commencing a 

 ' Frequency observation,' the tension was noted, the index was set to the 

 zero division on the paper, and the clock started ; the electrometer being at 

 the same instant discharged. As the index came successively over each 

 mark, the tension was observed by the electrometer and the scale reading re- 

 corded against the mark on the paper. These observations were continued 

 (for the most part) until, from the progression of the readings, the conductor 

 seemed to have acquired the full tension corresponding to the electrical state 

 of the air at the period. The inverse measure of frequency adopted is the 

 number of minutes between the time when the electrometer is discharged, 

 and the time when the readings of the electrometer have again attained to 

 one-half of their full amount. As the observations were in general continued 

 until the tension shown by the electrometer had reached its full amount, we 

 have thus a means of approximately allowing for the change in the electrical 

 tension of the air during the progress of the experiment. The numbers given 

 in the column ' Frequency,' are therefore, when the change of tension has not 

 been great, got in this way, viz. half the mean between the tension shown 

 before discharging the conductor, and that after the scale readings have again 

 become constant, is taken ; and the interval of time after discharge corre- 

 sponding to this reading is taken as the ' Frequency.' " 



In column K are contained the original readings of the mountain baro- 

 meter until January 9, 1851, and of the standard barometer after that time. 



In L the readings are those of the annexed thermometers, whose bulbs are 

 immersed in the mercury of the cisterns. 



In M the corrections of the mountain barometer embrace temperature 

 corrections as applied to glass scales, given in the Report of the Committee 

 of Physics of the Royal Society, which corrections were necessary, because 

 the brass scale does not extend to the cistern. " All the observations of this 

 instrument have been corrected for temperature, capacity and capillarity." 



" The observations" of the standard " are all corrected for temperature by 

 Schumacher's tables. No corrections have been applied for capillarity." 



It was found by fifty comparisons, made with due precautions, &c., that 

 " the mountain barometer read 0*02 inch higher than the standard. All the 

 observations previous to January 9, 1851, will therefore require a correction 

 of — 0*02 to make them comparable with those after that date." 



In N the readings are those of the Rutherford minimum and maximum 



.2 A 2 



