from an Increase of Temperature. 363 



rod to become drier ; but the whole of the inner case is so tight 

 as to prevent any evaporation, unless the upper pipe V, and the 

 under one of the same size, V, Fig. 1, which passes through the 

 bottom plate, are left open. The springs and roller represented 

 in this figure were firmly tied on the rods, and could easily be 

 shifted from the one to the other It was my intention to have 

 ascertained the expansion of the different rods between the freez- 

 ing and the boiling points, by first filling the space SS, Fig. 4$ 

 with melting ice, and then by passing a current of steam to get 

 the higher point, but the difficulty of getting ice until a few 

 weeks since made me abandon this ; besides, I found that, to 

 keep the thermometers above 208° F., sent a great heat through 

 the room, on account of the strong fire it was necessary to keep 

 up. As the thermometers were made with every precaution to 

 insure their accuracy, I contented myself with the range I could 

 get between the temperature of the room, which was generally 

 under 50°, and 207° or 208°. 



The rod was put into the pyrometer, the micrometers set, and 

 allowed to stand over the night, and the lowest point taken at the 

 height at which the thermometers stood next day, by making the 

 line on the under stud intersect the angle of the spider threads 

 of the lower micrometer ; and the index error of the line on the 

 upper stud was read from the zero of the upper micrometer : This 

 was repeated four times, by deranging and again adjusting the 

 height of the under line by means of the screw G. 



After completing the readings, the steam was turned into 

 the case, and the expansion of the rod measured by a similar 

 set of readings, as soon as the line on the upper stud was found 

 to be stationary, which sometimes required the temperature of 

 the pyrometer to be kept at the same degree for an hour and 

 a half. 



I thought that my application of steam for heating a pyro- 

 meter was the first time it had been done, as I could find no ac- 

 count of its having been formerly used in any of the descriptions 



VOL. XIII. PART II. 3 A 



