100 REPORT OF THE SECRETARY. 



mapping of the absorption lines in this region, and of subsidiary investigations con- 

 nected with these, comprises nearly 300 pages, uniform in size with Smithsonian 

 Contributions. It is illustrated by about thirty-five full-page or folded plates and 

 numerous text figures. The text contains full descriptions of the apparatus and 

 methods employed in mapping the infra-red solar spectrum by the aid of the bolom- 

 eter, and gives as a result of research the positions in deviation and in wave length, 

 as well as the relative intensities of over 700 absorption lines or bands between the 

 visible limit of the red at A (0.76//) and what is practically the limit of the solar 

 spectrum for terrestrial purposes at 6//. 



A chajjter is devoted to a discussion of the seasonal and other variations in terres- 

 trial absorption, which have been noted at many points in the infra-red spectrum. 

 These variations, and especially those of a seasonal character, to whicli attention has 

 been drawn in several of my earlier reports, form an extremely interesting and per- 

 haps imjiortant feature of these studies. 



It is greatly to be regretted that the very best photo-mechanical processes are 

 inadequate to justly reproduce the holographs, upon whose evidence the spectrum 

 lines are determined, and this I fear will prove a still more serious hindrance to the 

 publication of the photographic results of the eclipse expedition shortly to be 

 descril:)ed. This miavoidable inadequacy of I'epresentation is a very considerable 

 obstacle to the satisfactory publication of astronomical and, to a lesser extent, other 

 scientific results, in these days when tlie advance of science is increasingly connected 

 with the interpretation of delicate photographic detail. 



(2) Progress of the usual work of the Observatory. — The ordinary operations of the 

 Observatory have been much restricted by the two special lines of work already 

 alluded to, for it must be recalled that the present observing force consists of but two 

 persons, so that the long preparations for the eclipse and the reading and rereading 

 of the proof sheets of the Annals left little time for other work. 



As mentioned in my last report, designs were then entertained for constructing a 

 new galvanometer of the highest sensitiveness, with a support calculated tt) reduce to 

 the lowest limits the earth tremors which are so prejudicial in the use of such sensi- 

 tive instruments, and with a tliorough system of magnetic shielding to enable the 

 galvanometer to escajie the fluctuations of the magnetic field. This apparatus has 

 been finished, excepting the needle system of the galvanometer, and M^as tried in 

 January of this year with a temporary needle system of only 1.9 milligrams weight. 

 The result appears to be promising. As thus tried, with the galvanometer case par- 

 tially exhausted and a time of swing of about three seconds, no fluctuations of the 

 spot of light reflected from the galvanometer mirror exceeding 0.1°"° could be 

 observed on the scale 2 meters distant. The system of support and tlie magnetic 

 shielding appear, therefore, to be very perfect, and a few words describing them may 

 be appropriate here. 



System of supjwrt. — A massive i)ier rises through the basement room of the new 

 laboratory to the level of the ground floor. Upon this is a four-legged table 8 feet 

 high, whose top is a flat iron ring of about 500 pounds weight. Four iron cylinders 

 are fastened upon the top of the ring, and in these float on mercury smaller iron 

 cylinders supporting a second iron ring. From the floating ring descend three steel 

 wires, each 15 feet long, passing through holes in the capstone of the pier and sup- 

 porting a heavy cage of iron, in which is the galvanometer. The points of support 

 are at the plane of the center of gravity of the iron cage, and also in the i)lane of 

 the support of the galvanometer-needle system. Four dampers in glycerin baths 

 prevent torsional vibration of the cage. Thus the whole is a combination of four 

 methods of reducing the effects of earth tremors. First, a deep solid pier; second, 

 a "rickety table," supporting a weight of 1,600 pounds; third, a mercury flotation 

 of 1,000 pounds, whose center of gravity is below the mercury itself; fourth, a long 

 "Julius three-wire suspension," supporting 700 pounds. 



