80 Capt. Abney and Lieut.-Col. Festing. [Mar. 15, 



by transient vertical induction in soft iron is small, and of the same 

 value (nearly) for ships of similar construction. 



7. The preceding conclusions point to the conditions which should 

 govern the selection of a suitable position for the standard compass 

 with regard to surrounding iron in the ship. 



II. " Atmospheric Absorption in the Infra-Red of the Solar 

 Spectrum." By Captain Abney, R.E., F.R.S., and Lieut- 

 Colonel Festing, R.E. Received March 5, 1883. 



Any investigations on the subject of atmospheric absorption are of 

 such importance in the study of meteorology, that we have deemed it 

 advisable to present a preliminary notice of certain results obtained 

 by us, without waiting to present a more detailed account which will 

 be communicated at a future date. From 1874, when one of us com- 

 menced photographing the spectrum in the above region, till more 

 than a year ago, the extremely various manners in which the absorp- 

 tions took place caused considerable perplexity as to their origin, and 

 it was only after we had completed our paper on the absorption of 

 certain liquids,* that a clue to the phenomena was apparently found. 

 Since that time we have carefully watched the spectrum in relation to 

 atmospheric moisture, and we think that more than a year's observa- 

 tions in London, when taken in connexion with a month's work, at 

 an altitude of 8,500 feet on the Riffel, justify the conclusions we now 

 lay before the Society. 



A study of the map of the infra-red region of the solar spectrum, t 

 and more especially a new and much more complete one, which is 

 being prepared for presentation to the Royal Society by one of us, 

 shows that the spectrum in this part is traversed by absorption lines 

 of varying intensity. Besides these linear absorptions, photographs 

 taken on days of different atmospheric conditions, show banded 

 absorptions superposed over them. These latter are step by step 

 absorptions increasing in intensity as they approach the limit of the 

 spectrum at the least refrangible end. In the annexed diagram, J 

 fig. 4 shows the general appearance of this region up to X 10,000 on 

 a fairly dry day : the banded absorption is small, taking place princi- 

 pally between X 9420 and X 9800 : a trace of absorption is also visible 

 between X 8330 and k 9420. On a cold day, with a north-easterly 



* " The Influence of the Atomic Groupings of the Molecules of Organic Bodies 

 on their Absorption in the Infra-Reel Eegion of the Spectrum." " Phil. Trans.," 

 Part III, 1881. 



f " Phil. Trans.," 1880. 



X The lines shown in the diagram are merely reference lines, and hare nothing to 

 do with the absorptions under consideration. 



