298 Mr. G. A. Hemsalech and the Gomte de Gramont on 



4. Blown arc. — Air is blown through the arc, either with 



the mouth or otherwise, from the moment of striking 

 it until its extinction. 



5. Water arc, glycerine arc, etc. — Arcs through liquid 



films according to nature of liquid employed. 



§ 9. Results of observations. 



For taking the spectrograph^ records the image of the 

 arc was so adjusted on the slit as to allow of perfect differ- 

 entiation between the luminous radiations emitted at various 

 stages of the phenomenon. For this purpose the images of 

 the pole tips were made to coincide with the slit in such a 

 manner that the motion of the ima<>e corresponding to the 

 tip of the upper pole took place along the slit. Spectro- 

 grams obtained in this way permit of distinguishing at a glance 

 between the radiations emitted by the discharge through the 

 liquid film and those given out a little later during the 

 arcing stage. Since a single arc flash did not always suffice 

 to adequately impress the photographic plate a number of 

 flashes, generally from 7 to 30, were given in succession ; a 

 fresh drop of liquid was placed in position between the 

 electrodes before each flash. It was found that the images 

 of successive flashes were fairly accurately superposed upon 

 each other. This was indicated by the sharply defined 

 borders of the narrow spectrum near the stationary pole 

 corresponding to the liquid film phase. In fact the definition 

 of the spectra obtained in this way was generally such as to 

 admit of the measurement both of the thickness of the liquid 

 film and of the total length of the arc flash. 



The electric current used for producing the arc was 

 derived partly from a public supply service at 110 volts 

 (Paris) and partly from a battery of accumulators at 80 volts 

 (Manchester). The current strength varied generally 

 between 5 and 10 amperes. 



On account of the important role played by enhanced lines 

 in astrophysical problems, special care was given to the 

 investigation of that region of the spectrum which is most 

 generally used by astronomers — namely, the region between 

 X 3600 and X 7000 ; and it is mainly also for this reason that 

 in addition to an estimation of the relative intensities of the 

 lines their characters in either phase have been carefully 

 noted. In order to facilitate the tabulation of these results 

 the following numbers and signs have been made use of for 

 expressing relative intensities and characters. 



