356 



Mr. Louis Bell on the Absolute 



Date. 



0, 



1887. 



Dec. 16 







36 



+1-16 





16 







36 



+0-66 





16 







36 



+0-67 





19 







36 



+0-64 





19 . 







36 



+1-56 





19 







36 



+0-85 



1888. 



Jan. 1 2 







36 



-1-19 





12 







36 



-1-61 





12 







36 



-1-79 





14 







36 



-1-09 





14 







36 



-0-95 





14 







36 



-0-89 





19 







36 



-0-48 





19 







36 



-0-59 





19 







36 



-0-49 





20 







36 



+0-51 





20 







36 



-0-11 





20 







36 



+0-55 



The mean value, corrected as before for error of thermo- 

 meter, is 0=350 59' 59"-06 + 0"-15. 



The effect of this probable error is obviously the same as 

 in the case of grating III. The mean value of the semiangle 

 between the telescopes was 



= 6° 58' 31"'0. 



During the observations with grating III. the barometric 

 height reduced to the place of ooservation was very nearly 

 762 milliin., but during the work with grating IV. it was 

 phenomenally high, reaching an average value of 766 millim., 

 an amount so far from normal pressure as to render a small 

 correction necessary. 



The mean temperature during the observations with III. 

 was about 21° C, but in the case of IV. it averaged almost 

 exactly 20° C, varying at most only two or three degrees from 

 that figure. 



Measurement of the Gratings. 



The comparator on which this, the most important portion 

 of the research, was accomplished was the same one described 

 in my previous paper. It had, however, been improved in 

 several particulars. The platform carrying the standards had 

 been fitted with smooth rack-and-screw adjustments, and the 

 microscopes and micrometers were new. The illumination of 

 a grating under the power used, — two hundred and fifty 



