REPORT OF THE SECRETARY. 123 



puted sensitiveness, so that in this way the mechanical disturbances of sound 

 and ground tremors which reach the galvanometer case, being compelled 

 to influence a larger mass, will produce a less prejudicial effect upon the 

 needle. 



It has also been sought to reduce magnetic disturbances of the needle by 

 the system of magnetic shielding described in my last year's report. 



The application of these several devices has as yet proceeded only so far 

 as is described in what follows: Several different systems of only 0.0019 

 gram weight have been tried in the galvanometer with the arrangements 

 of support and shielding already described, but not with the air exhausted, 

 and it has been found that up to times of single swing of two seconds the 

 average accidental deflections on a scale at 3 meters do not exceed 0.1 

 millimeter, and the time of swing has actually been raised to ten seconds 

 without excessive disturbances. The effectiveness of exhaustion of the air 

 to make the sensitiveness proportional to the square of the time of swing 

 has been studied, and these studies though not complete indicate that for 

 air pressures of less than 1 millimeter of mercury this relation will be 

 approximately followed. 



A "heavy" needle system of 0.015 gram weight is in process of con- 

 struction, whose computed sensitiveness, it is believed, will equal or 

 slightly exceed that of the light systems already tried, while its steadiness 

 will be much greater. 



The most sensitive "light" needle system used gave at 1.5 seconds' single 

 swing in atmospheric pressure a deflection of 1 millimeter on a scale at 

 1 meter in a galvanometer of 1.4 ohms resistance with a current of 

 1(1(1(10 50 amperes. The damping was then so excessive that the sec- 

 ond swing was but y 1 ^ the magnitude of the first. If the hopes now reason- 

 ably entertained are realized the "heavy" needle can he effectively used 

 at ten seconds' single swing in vacuum, with a scale at 3 meters, and a 

 current of touoooottooooo amperes will in actual practice give a deflection 

 of 1 millimeter, and it is possible that a current of toooooo 2 ooooooij amperes 

 can be detected. Such a working sensitiveness as may thus be expected 

 would exceed that employed in taking the holographs of 1898 by 5,000 

 fold, taking into account the ratio of the galvanometer resistance employed. 

 The gain of working sensitiveness now actually attained in preliminary 

 experiments is no less than a hundredfold. If the fiftyfold further gain 

 hoped for is actually accomplished the field of research thus opened is 

 enormous, so that I regard these improvements in the galvanometer as 

 now the first consideration. It is greatly to be regretted, however, that 

 owing to the unfortunate situation of the observatory in the midst of city 

 disturbances the difficulties to be overcome are so large. In this connec- 

 tion, I venture to express the hope that the change of site of the observa- 

 tory contemplated in your former reports may some time be accomplished. 



Personal equation apparatus. — A portion of the time of the Junior Assist- 

 ant has been employed in the testing of an apparatus of your own design 

 to eliminate personal equation in time observations. These experiments 

 are not yet completed. 



Absorption of the solar atmospJiere.— An investigation of the absorption of 

 the solar radiation in the sun's atmosphere has been begun. A large solar 

 image is formed, and holographs are made at points near the center and 



