18 PROF. B. STEWART ON AN INSTRUMENT FOR 



The stem of the thermometer protrudes from the chamber 

 as in the figure, A screw Sj somewhat larger in diameter 

 than the bulb of the thermometer, is made use of to attach 

 the thermometer to its enclosure ; and a smaller screw, S', 

 pressing home upon india-rubber washers, enables the ther- 

 mometer to be properly adjusted, and kept tight when in 

 adjustment. 



In the present instrument the internal diameter of the 

 chamber is 2 inches, while the bulb of the thermometer is 

 about 1 5 inch in diameter. 



The scale of the thermometer is very open, more than 

 an inch going to one degree. I have generally allowed the 

 image of the sun given by the lens to heat the thermometer- 

 bulb for one minute, during which time an increase of tem- 

 perature not exceeding in any case two degrees has been 

 produced. 



As far as principle is concerned there appears to be no 

 objection to the present instrument ; nevertheless it is open 

 to a very serious practical objection. The scale being so 

 very open, the stem comprehends only a few degrees; 

 frequently, therefore, the temperature is such that the ex- 

 tremity of the mercurial column is either below or above 

 the stem. Now the thermometer has a small apper cham- 

 ber ; and by means of a method of manipulation well known 

 to those who work with thermometers, it is possible to add 

 to or take aM^ay from the main body of the mercury in the 

 bulb, so as to keep the end of the mercurial column always 

 in the stem. But experience has convinced me that for a 

 thermometer with such a large bulb, frequent manipulation 

 of this kind is not unattended with danger to the bulb. On 

 this account the instrument in its present form is, I conceive, 

 unsuited for steady work in an observatory from year to 

 year. 



It is, however, possible, without any appreciable sacrifice 



