HEAT OR CALORIC. 



61 



Difference between an air thermometer and a differential thermo- 

 meter, illustrated upon a large scale. 



" The adjoining figure represents an in- 

 strument, which acts as an air thermome- 

 ter, when the stopple S is removed from 

 the tubulure in the conical recipient R ; 

 because in that case, whenever the densi- 

 ty of the atmosphere varies either from 

 changes in temperature, or barometric 

 pressure, the extent of the alteration 

 will be indicated by an increase or di- 

 minution of the space occupied by the 

 air in the bulb B, and of course by a 

 corresponding movement of the liquid 

 in the stem T. But when the stopple 

 is in its place, the air cannot, within 

 either cavity of the instrument, be af- 

 fected by changes in atmospheric pres- 

 sure : nor can changes of temperature, 

 which operate equably on both cavities, 

 produce any movement in the liquid which 

 separates them. Hence, under these cir- 

 cumstances, the instrument is competent 

 to act only as a differential thermometer." 



Dr. Hare. 

 Self-registering thermometer. 



" This figure represents a self-registering 

 thermometer." 



" It comprises necessarily a mercurial and a 

 spirit thermometer, which differ from those or- 

 dinarily used, in having their stems horizontal, 

 and their bores round, also large enough to ad- 

 mit a cylinder of enamel, in the bore of the 

 spirit thermometer, and a cylinder of steel, in 

 the bore of the mercurial thermometer. Both 

 the cylinder of enamel and that of steel, must 

 be as nearly of the same diameters with the 

 perforations, in which they are respectively sit- 

 uated, as is consistent with their moving freely, 

 in obedience to gravity, or any gentle impulse." 



" In order to prepare the instrument for use, it 

 must be held in such a situation, as that the ena- 

 mel may subside as near to the end of the al- 

 coholic column as possible, yet still remaining 

 within this liquid." 



