Mr Harvey on tl<£ Rates of Chronometers. 173 



we regret our limits will not permit us to insert the results. Mr Harvey, 

 however, found, that a difference in the density of the air, represented by 

 a quantity less than an inch of quicksilver, if continued for a day, was ca- 

 pable of affecting all the Chronometers employed ; and this is an atmos- 

 pheric change by no means uncommon in this variable climate. Nor is it 

 indeed necessary, continues the author, that the alterations of density 

 should even continue for twenty-four hours, since, from the change of 

 rate being instantaneous, as he afterwards proves, six hours will be suffi- 

 cient, in some instances, to disclose it. In cases, however, where the va- 

 riations of the mercurial column are but small, and its transition from one 

 state to another marked by a gradual character, the effect on the generality 

 of Chronometers is scarcely if at all perceptible. 



With a difference in the mercurial column of an inch and three-quarters, 

 or two inches, Mr Harvey has little doubt but all time-keepers must be 

 influenced ; and it is moreover known, he remarks, that from a species of 

 reaction in the atmospherical columns, the greatest depression of the baro- 

 meter succeeds to a considerable elevation of it, and vice versa, so as to ex- 

 hibit a difference of this kind. Mr Harvey, accordingly, endeavoured to 

 obtain the rates of some good Chronometers during the remarkable depres- 

 sion of the barometer in December 1821, but without success; and he 

 farther observes, that there can be little doubt but, had the rates of some 

 good Chronometers been carefully attended to, during this singular altera- 

 tion of atmospheric density, variations of rate, at least equivalent to that 

 produced by transporting a time-keeper from London to Geneva, would 

 have been found. 



Another curious part of the paper is, the consideration of the question, 

 Whether the alterations of rate observed by Mr Harvey during his experi- 

 ments, were immediately acquired, the moment the change of pressure 

 took place, or whether it was an effect which the air gradually produced 

 on the machine. 



To determine this, a pocket and box Chronometer, possessing detached 

 rates of + 9."0, and + l."9 were placed under the receiver of the air 

 pump, in air denoted in density by 2 inches of the mercurial column ; and 

 which great degree of exhaustion was employed in order that, by produc- 

 ing considerable alterations of rate, the changes during very small intervals 

 of time might be perceptible. 



At the expiration of an hour, the increment produced in the rate of the 

 pocket Chronometer, by a mean of three observations, was found to be 

 + l."33 ; whereas the detached rate, in the same time, would have 

 amounted only to + 0."37, being a clear increase of ()."96 in consequence 

 of the diminished pressure. At the end of the second hour, the mean rate 

 was found to be + l."23, at the third l."35, at the fourth l."30 ; and the 

 observations were continued through the entire twenty-four hours, the 

 mean of the horary observations from noon to midnight being + l."12, 

 and the latter + l."10. The entire rate for the twenty-four hours 

 amounted to 4- 26."6, being an increase on its detached rate of 17. "6". By 

 a series of similar experiments with the box Chronometer, the mean of the 

 horary rates for the first twelve hours was + 0."92, and of the last 



