252 Temperature of Mercury in a Siphon Barometer. 



The first observation was made in a large unoccupied room wiiere 

 the instrument, having been suspended for some hours, indicated 

 a temperature of 31.1 degrees Fah. The instrument was then 

 quickly removed, together with the detached thermometer, to a 

 room at the temperature of 70 degrees, in which all the succeed- 

 ing observations were made. The second vertical column ex- 

 presses the time in minutes after the first observation. The 3d, 

 4th and 5th express in that order the temperatures of the detach- 

 ed and attached thermometers, and of the mercury in the tube, 

 calculated according to formula (1). The 6th and 7th columns 

 express respectively the number of degrees the temperature of 

 the mercury in the tube was behind the attached and detached 

 thermometers. From this table it appears that, after the barom- 

 eter had been suspended a quarter of an hour for example, the 

 detached thermometer errs as an index of the temperature of the 

 mercurial column by 7.4 degrees, and the attached thermometer 

 by 3.8 degrees ; and that the corresponding errors, after half an 

 hour's suspension, are 2°.4 and 1°.3 respectively. These errors 

 would have been somewhat diminished if the diameter of the 

 tube had been strictly equal to that of the barometric one ; but 

 on the other hand they might have been aggravated and rendered 

 more uncertain if the tube had been as long as the barometric 

 one, and had not the temperature of the room been sustained at 

 an uniform height. The observations of the table were made 

 with care under circumstances favorable to accuracy, and were 

 verified by three or four other tables, with which it substantially 

 agreed. From the frequency of the observations, some slight er- 

 rors of time may have been committed, but too inconsiderable to 

 affect the particular object of the experiment. 



We conclude therefore that the attached thermometer is an un- 

 certain index of the temperature of the mercurial column, except 

 in those rare cases when the temperature of the air is known to 

 be uniform, and not even then until after nearly an hour's sus- 

 pension of the barometer. 



