SOLAR ECLIPSE OE JANUARY 3, I908 — ABBOT 37 



These were inclosed in a wooden cylinder 7 centimeters in diameter 

 and 18 centimeters long, itself shaded by a ventilated double-walled 

 brass shield. A battery of 4 Gladstone-Lalande cells was used, fur- 

 nishing a current of 0.4 ampere. This battery was located in the 

 hut. and means for exactly balancing and trying the sensitiveness 

 of the bolometric circuit were provided by joining to one galvanom- 

 eter terminal and one battery terminal an adjustable resistance of 

 about 500 ohms, acting as a shunt around one of the 5-ohm coils. 

 It proved necessary to shade the copper cables connecting the bolom- 

 eter and the apparatus in the hut, but after this w^as done the 

 whole apparatus worked very satisfactorily, without prejudicial 

 drift or wiggle of the galvanometer spot. When considerable 

 changes of the pointing of the equatorial were made, it was generally 

 necessary to alter the balancing resistance slightly, as would be 

 expected in consideration of the changed inclination of the bolometer 

 strips. During the eclipse the time of single swing of the galvanom- 

 eter was 1.9 seconds, and a change of i ohm in the balancing re- 

 sistance produced 250 millimeters deflection. This indicates that a 

 rise of temperature of one bolometer strip of about 0.00001° C. 

 would have produced i millimeter deflection at that time. These, 

 of course, are far from the most sensitive conditions possible,' but 

 were regarded as good for a temporary installation. 



The attention of the reader is invited to the following improve- 

 ments in the apparatus of 1908 as compared with that of 1900: 



1. One mirror replaces seven. 



2. The uncertain exchange of radiations of long wave-length 

 between the bolometer and sky is eliminated by interposing glass. 



3. Each observation is limited to a comparatively small angular 

 area, well defined in position. 



4. An absorbing screen for indicating the quality of the rays is 



introduced- 



5. ]\Ieans are employed for comparing in intensity the rays of the 

 sun, the sky, and the corona. 



During the eclipse the writer was charged with pointing and 

 manipulating the equatorial, Mr. Moore with reading the galvanom- 

 eter, and Chief Yeoman Edward M. Chase, of the Annapolis, with 

 giving time signals and exposing two small cameras. 



' In Washington, with a scale distance of 4 meters and a time of single 

 swing in a vacuum galvanometer case of 7 seconds, a deflection of o.l milli- 

 meter has been measurable. This corresponded to a rise of temperature of 

 0.0000000 1. 



