Notices respecting New Books, 219 



contains the greatest amount of energy. This is in agreement 

 with the experimental results, for the " many-lined spectrum " 

 of mercury is obtained when a heavy discharge with capacity 

 in the circuit is passed through the vapour at a low pressure. 

 Since a definite minimum amount of energy may be expected 

 to be required for the production of any one form of positive 

 ion, it follows that the corresponding neutral system, and the 

 lines to which it gives rise, will appear when the energy is 

 increased up to that value ; and it is this which makes it 

 possible to obtain several distinct spectra with quite definite 

 changes from one to the other as the electrical conditions are 

 gradually altered. The new spectrum is not at its best when 

 first visible, but the new lines get brighter as further change 

 in the conditions of the discharge increases the number of 

 systems emitting them. If the conditions of the discharge 

 are such as to form several types of radiating centres at the 

 same time, the spectrum contains the lines due to all of them. 

 The " many-lined spectrum " observed by the author con- 

 tained all the lines of the other spectra with the exception of 

 five in the red and orange regions which were sometimes 

 seen, but were not visible when the spectrum was at its 

 best. 



XXI. Notices respecting New Books, 



Kecent publications of the U.S. Coast and Geodetic Survey include 

 "Besults of Observations made at the . . . Magnetic Observatory 

 at Cheltenham, Maryland, 1907&1908, by Daniel L. Hazard," 

 and "Arctic Tides by Eollin A. Harius" (Washington, 1911). 



r FHE former publication follows the same general lines as earlier 

 - 1 - volumes of the series, but the copies of disturbed magnetic curves 

 are allowed a time-scale of 15 mm. to the hour instead of 9 mm. as 

 previously. This is a marked improvement, as it allows details to 

 be followed with much greater ease and accuracy. 



The second volume deals in the first instance 'with tidal obser- 

 vations along the north coast of Grant Land and Greenland made 

 by the Peary Arctic Expedition of 1908-9, but collects for com- 

 parison practically all available results relating to tides in high 

 northern latitudes. A table is given of the harmonic tidal con- 

 stants at all stations north of 60° for which the author could find 

 data. There is a discussion of the phenomena and some criticism 

 of Dr. Xansen's views. The volume merits the attention of all 

 interested in tidal analysis. 



