202 Dr. Bostock's Observations on [March, 



difference in the weight of the atmosphere. We have two 

 registers kept by the makers of instruments, Mr. Caiy and Mr. 

 Banks, both residing in the Strand; but unfortunately these can 

 not be compared together with perfect accuracy, because the 

 observations are taken at different periods of the day — Mr. 

 Cary's at one o'clock, and Mr. Banks's at nine. But this 

 difficulty may, in part, be removed by comparing each of these 

 diaries with that kept by the Royal Society in Somerset-house, 

 where it must be supposed that the instrument would be of the 

 most perfect kind, and where any derangement that might have 

 occurred by its removal must have been detected and rectified. 

 The observations at Somerset-house are made twice in the day ; 

 the time is not always the same, but it is specified at each ob- 

 servation ; the morning observation is generally between eight 

 and nine, and the afternoon observation between three and four. 

 I therefore took the afternoon observation, and compared it with 

 Mr. Cary's, arranging them, as before, in the form of a table. 

 (See No. 4.) 



The difference between the lines described by these two 

 barometers is less than any which we have yet examined ; but 

 still the same kind of variations will be found to exist. And 

 here we may observe, as in the former examples, that the actual 

 height of the mercury is not in proportion to its elevation above 

 the level of the ocean ; for although the two barometers are 

 placed at nearly the same elevation, Mr. Cary's generally indicated 

 a less degree of atmospherical pressure ; but, as in the other 

 cases, this difference was not uniform, and occasionally Mr. 

 Carey's barometer became the higher of the two. We also 

 perceive that in any great variations of the mercury the amount 

 of the change was not the same in both instruments ; and what 

 is still more singular, they did not always attain their maximum 

 elevation or depression on the same day. We must indeed bear in 

 mind that the observations were not perfectly contemporary ; 

 but I believe every one will admit that this circumstance will not 

 account for all the difference between them. 



So far as the time of the observation is concerned, we are 

 able to obtain more accurate results by comparing the diaiy of 

 Mr. Banks with that of the Royal Society ; and in order to 

 render the comparison quite unexceptionable, I have only 

 selected those days on which the observations at Somerset- 

 house were made precisely at nine, the hour at which Mr. 

 Banks always examined his barometer. As I was on this 

 account obliged to omit several days, when the observations were 

 not taken precisely at the same hour, the comparative heights of 

 the instruments are not marked, as before, by a continued line ; 

 but the two registers are placed in parallel columns, to which a 

 third is added stating the amount of the difference between 

 them. 



