660 : REPORT—1899. 
3. A discussion on platinum thermometry was opened by the reading 
of papers by Professor H. L. Catnenpar, F.R.S8., and by Dr. P. Cuappuis 
and Dr. J. A. Harker, which are appended to the Report on Electrical 
Standards, p. 242. 
WEDNESDAY, SEPTEMBER 20. 
The following Papers were read :— 
1. Recent Magnetic Work in North America. By L. A. Bausn, Chief of 
Division of Terrestrial Magnetism of U.S. Coast and Geodetic Survey. 
1. An account of the recent magnetic work carried out by the United States 
Coast and Geodetic Survey in various parts of the United States and Alaska, and a 
general outline of the proposed more detailed work were given. The General 
Government, through the Coast and Geodetic Survey, has recently made prepara— 
tions for having a detailed magnetic survey made of its possessions, the general 
scheme of which is to be completed in 10 to 15 years. Observations on ocean areas are 
likewise tobemade. In thegeneral scheme the stations are to be about 30to35 miles 
apart. After the completion of the general survey, stations will be added in: 
regionally disturbed areas. The desirability that Canada and Mexico will likewise: 
undertake at the same time similar surveys was set forth. 
2. The general results of the recent magnetic survey of Maryland, made by the: 
author, under the auspices of the Maryland Geological Survey and the Coast and 
Geodetic Survey, were briefly laid before the Association and charts exhibited, 
showing the isogonic, the isoclinic, and the isodynamic lines over Maryland for the- 
epoch January 1, 1900. Areas of marked regional disturbances were mapped out 
on a special chart. The secondary residual field of the earth’s magnetism, 7.e. the: 
portion remaining after deducting the uniform magnetisation, was represented 
graphically for the year 1900, 
2. The Spectral Sensitiveness of Mercury Vapour in an Atmosphere of 
Hydrogen, and its Influence on the Spectrum of the latter. By 
E. Percrvat Lewis, PA.D.! 
An account was given of some experiments carried on in the University of 
Berlin. The spectrum of hydrogen at different pressures, contained in vacuum 
tubes with external electrodes, first pure and then containing varying quantities 
of mercury vapour, was examined photometrically with the following results :— 
1. A quantity of mercury vapour corresponding to the saturation density 
of —5° produced a spectrum of measurable intensity, but did not appreciably affect 
the intensity of the hydrogen spectrum. 
2. A quantity of mercury vapour corresponding to the saturation density at 
21° produced a diminution of one-half or more in the intensity of the entire 
visible hydrogen spectrum. 
3. The intensity of the mercury spectrum seemed proportional to the amount 
of its vapour present. 
4, The visible radiant energy of the hydrogen and the mercury seemed to be 
divided between them in the proportion of their relative quantities in the 
mixture, 
The weakening of the hydrogen spectrum by the presence of mercury cap 
1 See Wied. Ann. 1899, vol. 69, p. 398. 
