436 THE POPULAR SCIENCE MONTHLY 



Because of its high magnetic permeability, iron is used to shield the 

 suspended system of magnets from variations in the magnetic field, such 

 as produced by passing street cars, etc. This galvanometer is very 

 sensitive, so that it responds to a current of less than one ten-billionth 



Fig. 2. Showing the Glass Receptacle of Fig. 1 Mounted by Means of the 

 Screws S, S, in the Plateholder of the Crossley Reflector. 



of an ampere, and in observing the heating effect produced by different 

 stars, measurements could be made in four to five seconds. 



This outfit consisting of an ironclad galvanometer and two re- 

 ceptacles containing the thermocouples was constructed in Washington, 

 D. C, and carried to Mt. Hamilton, Calif., a distance of over 3,200 

 miles without serious mishap (one thermocouple was broken in climbing 

 the mountain). A vacuum pump was shipped, but it was not un- 

 packed. The slight vapors given off from the cement and stop-cock 

 grease were removed from these receptables by occasionally heating the 

 calcium by means of a small alcohol blast lamp. From this it is evi- 

 dent that one of the principal achievements was in demonstrating that 

 with an equipment of several thermoelements, mounted in evacuated 

 receptacles, one can go to the remotest station for radiation measure- 

 ments, without taking an expensive or cumbersome vacuum pump. 



After observing for several nights it was found that red stars 

 emitted far more radiation than do the blue ones having the same 

 photometric brightness, and attention was given mainly to the solution 

 of this question. Accordingly stars were selected having the same 

 visual magnitude, but differing in color ; and which were close together 

 in right ascension and zenith distance, in order to obtain the measure- 

 ments of the ways traversing the same air mass. In passing through 

 our atmosphere, the radiations from the red stars suffer a greater 

 fibsorption than do the blue stars. For this reason it was desirable to 

 eliminate, as much as was possible, the effect of air mass. 



As the work progressed, it became a rather instinctive feeling to 



