4 



Lieut. J. H. Hennessey on the [June 16, 



understanding the Committee to suggest that I should adopt Kirchhoff's 

 map and scale as a basis, it became incumbent on me to magnify the 

 image actually viewed some three times before sketching it on my map. 

 As an aid towards this end, I removed the strong metal cross wires in the 

 telescope, and replaced them by two tolerably stout silk fibres, parallel to 

 one another, and rather more apart than the image of B sun high. I 

 could thus compare a given line with one of the fibres, or, where more 

 convenient, with the space between them ; and, through the intervention 

 of the fibres, compare any one line with any other. These comparisons 

 of magnitude were thus all mental, there being no micrometer in the eye- 

 piece to measure small spaces with, and the smallest quantity that I could 

 measure on the circle being the distance between the two lines in D. In 

 selecting a relative unit between Kirchhoff's and my map, I have adopted 

 the breadth he has assigned to the line A. This line will therefore be 

 found equally wide in his map and in mine at sunset. The necessity for 

 enlarging the image leads to an unintentional deception, deserving notice. 

 It will be seen that my line A, for instance, is equally and intensely black 

 throughout ; whereas the same line, shown at the same width in Kirch- 

 hoff's map (402 to 407), is made up of several lines of varying intensity. 

 This difference is due to the lesser power of my instrument ; and in all 

 probability the line up here would decompose into its component lines 

 under greater dispersion. 



11. Necessity for constructing my map from independent measures. — 

 The want of intensity generally in the spectrum sun high at Mussoorie, 

 combined with the smaller power of the instrument, made it exceedingly 

 difficult, and in most cases impossible, to identify individual lines in 

 Kirchhoff's map with corresponding ones under view ; so that, after 

 making every endeavour at identification, I was obliged to content myself 

 with adopting the positions (sun high) assigned by him to the strong lines 

 A, B, C, and D, and to place all the other lines of sensible intensity by 

 means of differential measures and interpolation. Practically speaking, 

 this amounted to the construction of a new map, so far as my wants were 

 concerned. 



1 2. Definitions. — By a constant line is intended one that presents the 

 same appearance at sunset or sun high ; a variable grows darker at sunset, 

 and, generally speaking, widens out ; and an atmospheric or air-line is 

 invisible only sun high. By a band may be understood those broad belts 

 which suddenly appear like shadows at sunset ; instances of bands occur 

 on both sides of C and elsewhere. 



13. Lines mapped. — Every variable or air-line (or band) sensibly visible 

 has been measured and mapped ; but of the constant lines only those 

 sufficiently intense to be easily intersected were observed and placed. It 

 appeared undesirable to crowd the map with a large number of faint lines, 

 whose property of constancy made their presence in an atmospheric map 

 redundant. On the other hand, the introduction of certain constant and 



