HEAT OR CALORIC. 



55 



by the variations of atmospheric pressure as well as by thermomet- 

 rical changes. Dr. Hare. 



(b.) Leslie's differential thermometer. For the construction of this 

 instrument, a ball is blown at each end of a glass tube bent twice at 

 right angles. 



The tube contains usually sulphuric acid colored by carmine the 

 balls contain air, which, as well as the contained fluid has no com- 

 munication with the atmosphere. 



(c.) It indicates only the difference of temperature between the two 

 balls. It is very useful in delicate experiments on heat, where the 

 variations of temperature are minute. 



(d.) Howards improvement of Leslie's thermometer. 



Dr. Howard of Baltimore,* has substituted ether for the sulphuric 

 acid the ether is boiling when the instrument is sealed, and therefore 

 there is a vacuum over the fluid, except that the space is filled with 

 the vapor of ether ; this instrument is vastly more sensible than Les* 

 lie's original one, and with it the heat was believed to be discovered in 

 the moon's rays by Dr. Howard. f 



DIFFERENTIAL THERMOMETER. 



o o 



This instrument consists of a glass tube 

 nearly in the form of the letter U, with a bulb 

 at each termination. In the bore of the 

 tube there is some colored liquid, as for in- 

 stance, sulphuric acid, alcohol, or ether. 

 When such an instrument is exposed to any 

 general alteration of temperature in the sur- 

 rounding medium, as in the case of a change 

 of weather, both bulbs being equally affected, 

 there is no movement produced in the fluid ; 

 but the opposite is true, when the slightest 

 imaginable calorific influence exclusively af- 

 fects one of the bulbs. Any small bodies, 

 situated at different places in the same apart- 

 ment warmed by a fire, will show a diversity 

 of temperature, when severally applied to the 

 different bulbs. Dr. Hare. 



2. CONSTRUCTION OF THE COMMON THERMOMETER. 



Sa.) Take a glass tube, of uniform bore, sealed at the glass-house. 

 ts uniformity is ascertained by introducing a little mercury, and 



*Lond. Quar. Sci. Jour. Vol. 8. pa. 219. 



t Am. Jour. Vol. II, pa. 329, 



