OF ARTS AND SCIENCES. 43 



and that the scale was correct at 0° C, which was certainly quite 

 nearly true. The value of the coetFicient of expansion of air found 

 with this apparatus was a = 0.003679. 



In February, 1879, I made a comparison of four thermometers, 

 including Baudin 7335 and GIGI, and two other thermometers of little 

 value, at temperatures from 100° to 250° C, with the apparatus just 

 described. The results were fairly good, but not entirely satisfactory. 

 Later in the same year. Dr. E. H. Hall (then a student at Baltimore) 

 made a similar series of comparisons, using Baudin thermometers 

 Nos. 6161, 7315, 7325, and 7324. His results with No. 6161 differed 

 somewhat from mine, owing chiefly to the progressive change in zero 

 which attended this thermometer during my own use of it, as well as 

 to something of the same change during his own measurements. The 

 results as a whole agree fairly well, and I have deemed it best to 

 average the corrections found on all the Baudin instruments of both 

 series, and to use this average result as a correction to Baudin 7335, 

 which I employed in my subsequent measurements. This seemed to 

 me the safest course, because of the general agreement of all the 

 Baudin thermometers used, and because more comparisons of 7335 

 could not be made at that time to eliminate the errors entering into a 

 single series. That the table of errors thus obtained and given below 

 is not all that might be desired is obvious, but I am disposed to believe 

 that at 200° it is not in error by an amount exceeding 0°.5, and by a 

 less amount at lower temperatures, thus introducing into y an error 

 not exceeding 0.1 or 0.2 per cent. 



TABLE XII. 

 Deviation of Baudin No. 7335 from Air Thermometer. 



At A At A 



The measurements made at 225° were taken with Baudin No. 7789, 

 and the correction to be applied to it was ascertained, by a compari- 

 son with No. 7335, to be -f0°.8. 



I have devised and had constructed an apparatus for air-thermometer 

 corrections at high temperatures, and had hoped to be able to make a 

 further series of comparisons of all thermometers used, but for reasons 

 elsewhere stated the work is unfinished, and has no prospect of imme- 

 diate completion. 



