Observations xvitJi Kater's Penduluuis. 165 



abandoned. For all the remaining swings, the times of D 

 and R were observed by the Shelton ; the D taking place 

 between an even and odd second, and R between an odd and 

 even second. If the size of the segment of the disc, when last 

 seen before Z>, was larger than the segment when first seen 

 after R, one second was added to the even second of time 

 immediately preceding Z>, and if the opposite occurred, one 

 second was subtracted from the odd second of time immediately 

 following R. This refinement was not necessary ; but as the 

 method required only a very little more attention it was followed 

 throughout. 



Temperature. 



The temperature of the pendulum was frequently observed 

 during each swing, and was shown by the two thermometers 

 Fahr. Nos. 667 and 668, placed on the dummy pendulum inside 

 the cylinder. Tiie adopted correction to the mean reading of the 

 two, was — 0-°13. 



During the first few swings the gas was allowed to burn in 

 the room ; but this brought about irregular changes of tempera- 

 ture, and caused the upper thermometer to read higher than the 

 lower, often by as much as 0-4 degrees. The gaslight was 

 accordingly discarded. For the rest of the time, till the con- 

 clusion of the observations, the temperature in the room did not 

 vary by as much as one degree Fahr., and the two thermometers 

 read always nearly alike. 



Two Richard's thermographs, ISTo. 1577 and No. 3131, were 

 placed close to the cylinder, giving a continuous record of the 

 temperature. During the first twelve swings the changes were so 

 irregular and large, amounting sometimes to 3° or 4° Fahr., that 

 curves had to be formed and intergrated in order to obtain a 

 satisfactory mean ; but for the remaining swings, the mean of 

 the observed temperatures at commencement and at the end of 

 the swing, as shown by the two thermometers on the dummy 

 with the correction — 0-13 applied, was adopted as representing 

 with sufficient accuracy the mean temperature of the pendulum. 



