86 



On Ascertaining the 



from D to E is occupied by mercury, and the remainder of 

 the bulb E F Is a Torricellian vacuum, the tube being hermeti- 

 cally sealed at both ends to exclude barometric influence. 



The effect of heat will be to expand the air in tlie bulb 

 and by the increase of its pressure to force the mercury out 

 of the bulb C D into E F, the upper part of which being 

 vacuum will offer no resistance. The weight being thus 

 removed nearer to the centre of oscillation, will be tanta- 

 mount to a shortening of the pendulum, and will cause it to 

 vibrate more rapidly, and in exact proportion to the tem- 

 perature, as it is well known that the expansion of air is uni- 

 form with every increment of temperature. 



A drop of oil on the surface of the mercury in C D, and 

 the substitution of hydrogen gas for air in B would probably 

 add to the perfection of the instrument ; — the object of the 

 oil being to prevent the transfer of air into the vacuum, and 

 the hydrogen to obviate the action of common air on the oil. 



By a slight and obvious modification of this instrument a 

 sensitive air thermometer may be constructed. 



It is, however, obvious that the true mean temperature Is 

 not necessarily the same as the mean of the two extremes of 

 the twenty-four hours. The thermometer may have been for 

 many hours near its highest point, and for a short time only 

 near its lowest, or vice versa. The force of this objection 

 becomes considerable in a climate like this, where a change of 

 temperature to the extent of thirty degrees is not unfrequently 

 known to occur during a single hour. Admitting, therefore, 

 the value of Six's thermometer, as registering in a most con- 

 venient manner the extremes of heat and cold, it cannot, I 

 think, with propriety be depended upon, as an instrument 

 affording data from which to calculate mean temperatures, 

 except approximately ; and this approximation may be 

 seriously remote from the exactness which modern science 

 demands. 



I now proceed to describe the principle of the instrument 

 which I propose for ascertaining mean temperatures. It is 

 well known that the pendidum of a clock vibrates more or 

 less rapidly according to its length; that the pendulum is 

 elongated by heat and shortened by cold ; and consequently, 

 that an ordinary clock has a tendency to go slower or lose 

 time in warm weather, and to gain time In cold. The effect 

 of slight changes of temperature upon an ordinary pendulum 

 is very inconsiderable ; but, if we can succeed in constructing 

 a pendulum, which shall be highly sensitive of heat and cold, 



