58 Mr. F. D. Brown's Notes on Thermometry. 



reproduction when lost or destroyed. With this end in view, 

 I made two thermometers at different times, and wholly 

 independently one of the other, and compared their readings. 

 To those who may wish at any time to construct a mercurial 

 thermometer without the elaborate appliances ordinarily em- 

 ployed, but in which absolute confidence may be placed, the 

 following details may be of interest: — 



A capillary tube of medium bore, about 800 millimetres 

 long, free from all flaws, and having as uniform a section as 

 possible, is provided with a millimetre-scale of 600 divisions. 

 The etching of this scale is a matter of great consequence : it 

 very frequently happens that the divisions on glass tubes are 

 not of exactly equal length, but that, owing to some defect in 

 the dividing-engine or some movement of the tube while un- 

 dergoing the process of division, some of the divisions are so 

 much longer or shorter than the rest as seriously to interfere 

 with the subsequent process of calibration. Even when all 

 the lines are equidistant, they are often so thick, and present 

 so irregular an outline when viewed through a telescope, that 

 it is impossible to fix upon any particular point as that repre- 

 sented by the dividing-line. The tubes I employed were 

 selected and divided with special care by Mr. Casella, the 

 lines being perfectly straight, less than 0-4 millim. in thickness, 

 and in all cases equidistant. 



As a glass tube, however carefully selected, is never of 

 uniform bore, it is necessary to ascertain the relative capacities 

 of the several divisions of the tube, or, in other words, to 

 " calibrate " it. As is well known, this is easily done by 

 placing a thread of mercury in successive positions along the 

 tube and observing its length, the mean capacity of the divi- 

 sions occupied by the thread being, of course, inversely pro- 

 portional to that length. In this way, and by adopting the 

 plan of correcting the position of the thread suggested by 

 Dr. Mills in the paper above referred to, which plan he had 

 been kind enough previously to communicate to me privately, 

 a table is readily constructed showing the volume of the tube 

 from the line marked to any line marked n, and also the 

 value of the succeeding division. The only difficulty connected 

 with this process is the accurate measurement of the length of 

 the thread of mercury in its several positions. It is true that 

 this may easily be done with a dividing-engine or some similar 

 instrument, such as a cathetometer provided with a micrometer 

 eyepiece and placed horizontally. As, however, reliable instru- 

 ments of this class are exceedingly costly, I designed a small 

 piece of apparatus for the purpose, which has proved so con- 

 venient and useful that 1 venture to describe it here. 



