OF ARTS AND SCIENCES : JANUARY 9, 1866. 59 



between the two D lines, there was a faint but broad nebulous band, 

 barely resolvable into lines of still smaller magnitude.* It is impos- 

 sible to represent this band accurately with a woodcut ; and the shaded 

 broad band marked k on the right-hand side of Fig. 4 only serves 

 to indicate its position and approximate breadth. 



The 26th of December was also a warm day for the season, with a 

 brilliapt sun. At one o'clock, P. M., the dry-bulb thermometer marked 

 55°, the wet-bulb 50°, and hence the amount of moisture in the atmos- 

 phere was 3.76 grains per cubic foot. The appearance of the D line 

 at this time is shown in Fig. 3. Two of the Hnes, rj and 6, and the 

 nebulous band k, seen on the 17th of November, were invisible, and 

 moreover the group of three lines 8 e f on the left-hand side of the 

 figure were only just within the limits of visibility. 



On the 25th of December only two lines were visible within the D 

 line, marked a and /3, in Fig. 2, and the last of these was quite faint. 

 The temperature at the time of observation was 46° ; the wet-bulb 

 thermometer indicated 40°, and the amount of moisture in the air was 

 2.42 grains per cubic foot. The sky was clear and the sun brilliant. 

 Lastly, on January 5th, 1866, one of the clear cold days which are 

 so common in our climate during the winter, only the single line a 

 was visible within the D line, as is shown in Fig. 1. At the time of 

 observation, near noon, the dry-bulb thermometer marked 10°, the wet- 

 bulb 9°, and hence the amount of moisture in the atmosphere was only 

 0.81 of a grain per cubic foot. The sun, liowever, was as brilliant as 

 in either of the previous cases. The D line also appeared as in Fig. 1 

 on the 8th of January, 1866, when the thermometer at noon stood at 

 10° below zero Farenheit, and when the barometer attained the un- 

 exampled height of 31 inches. 



The above figures have been drawn so as to show, as nearly as pos- 

 sible, the relative intensity of the different lines under different atmos- 

 pheric conditions. As no accurate means of making the comparison 

 are yet known, I was obliged to depend upon my eye alone, and small 

 differences at different times of observation may easily have escaped 

 my notice. Indeed, I should have been liable to great error, were it 

 not for the fact that one of the lines within the D line, marked a in all 

 the figures, does not vary in intensity, and served as a constant stand- 

 ard in making the observations. This is the only line which is given 



* We use this word in the same sense in which it is used by astronomers with 

 reference to the fixed stars. 



