272 Mr. L. Bell on the Absolute 



II. were not taken up until early in the succeeding March. 

 Precisely the same method of observation was employed, and 

 the results were nearly as consistent and satisfactory. 



The observations on the various davs were as follows : — 



Date, 1886. N * ml ? er of 

 ' beries. 



Angle. 



March 6 



. 2 



42° 5' l"-2 



10 . . 



1 



42 4 58 -6 



11 . . 



7- 



42 5 1-4 



15 . . 



1 



42 5 4-0 



16 . . 



6 



42 4 57 -8 



17 . . 



6 



42 4 58 -5 



18 . . 



7 



42 4 59 -1 



23 . . 



6 



42 4 58 -3 



When collected thus by days, the observations do not 

 appear to agree nearly as well as those made with grating I., 

 particularly since a solitary wild reading, that of March 15, 

 is retained. The distribution of the various readings, how- 

 ever, is such that, after weighing and combining, the final 

 result is by no means deficient in accuracy. It is 



= 42° 4' 59"-28±0"-2. 



The above probable error amounts to about one part in six 

 hundred thousand. The observations with grating II. were 

 uniformly in the fourth order of spectrum. 



Throughout the measurements with both gratings, the tem- 

 perature was kept uniform within a few degrees. 20° C. had 

 been selected as the standard temperature, and the variation 

 was rarely more than two or three degrees on either side of 

 that figure. The question of temperature determination is a 

 serious one in case of glass gratings; for it is very hard to 

 tell what heating effect the incident beam has on the grating, 

 and equally hard to measure that effect. It is hardly safe, 

 without extraordinary precautions, to assume that the grating 

 has the same temperature as the air near, and it is such a 

 bad conductor that it would not easily assume the tempera- 

 ture of the apparatus. In these experiments a sort of com- 

 promise was effected. A small thermometer was attached to 

 the thin metallic slip that held the edge of the grating, and 

 shielded by cotton from air-currents, which of course would 

 affect it much more than they would the grating. The 

 thermometer was a small Fahrenheit graduated to quarter 

 degrees, and quite sensitive. It was carefully compared, 

 throughout the range of temperatures employed, with thermo- 

 meter Baudin 7312, which served as a standard in all the 



