104 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



Hala. The supervisors of the Los Angeles County Park have cor- 

 dially assisted in the transfer, giving rights of occupancy, and ex- 

 tending the auto road quite to the doors of the proposed observatory, 

 without expense to the Smithsonian Institution. It is expected to 

 occupy Table Mountain beginning about October 1, 1925, Mr, 

 Moore's energetic efforts in the preliminary arrangements and the 

 preparation of buildings deserve high praise, ' ' 



An expedition under Doctor Abbot occupied Mount Wilson in the 

 summer and autumn of 1924. The solar cooker was rebuilt, as far 

 as concerned its oven, its circulatory system for hot oil, and its in- 

 sidation against heat losses. The new oil system Avas perfectly suc- 

 cessful in avoiding all leaks, such as always hitherto have marred 

 the operations. Also, the introduction of a larger reservoir, and 

 especially of " Silocel," or diatomaceous bricks, for heat insulation 

 proved highly satisfactory. The experiment was tried of intro- 

 ducing forced oil circulation by means of a little steam engine 

 operated by the heat of the reservoir. This worked Avell mechani- 

 cally, but proved unnecessary, as no higher temperatures of the 

 oven were reached when forced circulation was in operation. 



It was intended to use a vacuum jacket about the heater tube, but 

 the apparatus was not received in season. Without this crowning 

 improvement the solar cooker worked fully as well as in 1920, when 

 its reputation was first made, despite the fact that somewhat thicker 

 insulation of the reservoir is needed, as the cooling curve shows. 

 When this, and also the vacuum jacket, are applied, the machine 

 should be highly satisfactory. 



As noted in last year's report, the Fabry type of apparatus has 

 been installed on Mount Wilson to measure the quantit}' of atmos- 

 pheric ozone. This feeble constituent of the very high air is, we 

 believe, very important in the economy of the earth's heat, as well 

 as a fatal bar to observation of the most interesting part of the 

 spectra of the sun and the hotter stars. 



Having fully developed and tested the ozone outfit, photographic 

 solar spectra of the ozone-absorption region of the ultra-violet were 

 obtained in August, September, and October, 1925, Unfortunately 

 the great forest fire east of Mount Wilson cut off a good many other- 

 wise favorable days. By the generosity of Mr, Roebling a copy of 

 the Moll spectrophotometer for measuring the plates has been pro- 

 cured from A, Hilger. The reductions are not yet made. Mr. 

 Roebling's interest in this ozone research is so great that he has 

 made a grant to enable Doctor Fabr}- himself to continue daily 

 ozone measurements in France during a part of the year 1925. 



The importance of studies of the variation of the sun's output of 

 ultra-violet rays grows upon our attention. Not only the attack 



