406 



nexion of the air within the instrument with the external at- 

 mosphere is cut off; but when one of the balls is heated, 

 and the elasticity of the air within it thus augmented, and 

 the intervening column of fluid driven towards the cool ball, 

 the elasticity of the air within the latter is also obviously in- 

 creased by compression. Equal increments of temperature 

 cannot, therefore, produce equal augmentations of volume, 

 and, when the stem of such an instrument is divided into 

 parts of equal capacity, the corresponding temperatures 

 constitute, not an arithmetical series, but one which in- 

 creases much more quickly. In fact, if t be the tem- 

 perature, V the volume of the air in the heated ball, and 

 V the volume of air in the cool ball, t varies not as v, 



but as -. 



V 



This source of embarrassment in the air thermometer 

 Mr. Grimshaw removes by a contrivance, a notion of which 

 may be simply conveyed by describing his instrument 

 as a differential thermometer, in the cool ball of which 

 is placed a barometer, while to the side of the same ball a 

 little syringe is attached, by means of which air may be 

 pumped in or out, and the elasticity of the included air thus 

 rendered invariably the same, before the temperature (exhi- 

 bited upon the scale of equal parts attached to the stem in 

 connexion with the hot ball) is registered. Two forms 

 of this instrument were placed on the table of the Academy, 

 which, however. Dr. Apjohn stated should be considered 

 rather as rough models, than as finished instruments. Dr. 

 Apjohn observed, that Mr. Grimshaw intended attaching 

 to his thermometer a provision for keeping the barometer 

 vertical ; and marking upon this latter instrument two addi- 

 tional points of constant pressure, — one higher, the other 

 lower, than the atmospheric standard, — by the use of which, 

 when necessary, the scale of the instrument may be greatly 

 extended, so as to comprehend with ease the entire of the 

 atmospheric range of temperature. 



