PHILOSOPHICAL SOCIETY OF WASHINGTON. H 



instruments of the most approved character. The special class 

 of observations called for were made by Prof. C. A. Young;, of 

 Dartmouth College, with the 12-foot equatorial and spectroscopic 

 apparatus belonging to that institution. 



The astronomical station was on the high ground near Sherman 

 depot, which is situated on the line of the Union Pacific Railway 

 where it crosses the summit of the Black Hills, the most eastern 

 range of the Rocky Mountains. The latitude was 41^ 07' 49". 5; 

 the longitude west from Greenwich, 105° 23' 11"; and height 

 above the sea-level, 8335 feet. 



Three temporary observatories were erected ; one for the transit 

 and latitude instrument, chronograph and telegraphic apparatus; 

 a second for the meteorological instruments and observers; and 

 the third and largest for the equatorial telescope. The observa- 

 tions were commenced in June and continued through July and 

 part of August, the meteorological class having been made hourly, 

 day and night, for a term of sixty days. 



The longitude was determined by telegraphic exchange of 

 clock-beats, on seven nights, with Assistant A. T. Mosraan at 

 Salt Lake City, Utah Territory, the intermediate distam-e being 

 500 miles. The latitude was determined by 110 observations on 

 nineteen pairs of stars for differences of zenith distance. 



Gi^tei'n Barometers. — ■ 

 The highest mean of 24 hours was on June 21st, 22,488 

 The lowest mean " " June 30th, 21.977 



Mean height of the 60 day set . . . 22.281 



The tropical hours were found to be 3 A. M. and 5 P, M. for 

 the periods of daily minima, and 9 A. M. and 10 P. M. for the 

 daily maxima. The hourly oscillations at night were not half so 

 great as during the day. The barometric column was in all cases 

 reduced to the freezing point. 



Fahrenheit Thermometers. — The temperature of the air varied 

 from 32^ to 81°. 6, the mean of the day and night being 56^5. 

 The hour of lowest temperature was 4 A. M., or about half an 

 hour before sunrise, and of Jhe highest at noon. The mean tem- 

 perature of the night, between sunset and sunrise, was 50^3, and 

 of the day, 60°. 2. 



Solar Maximum Badiation Tliermometers. — The maximum 

 excess of temperature of the solar rays, divested of the effects of 

 vapor and wind, above the terpperatui'e as given by the thermom- 

 eters in the shade but otherwise freely exposed, was tolerably 

 uniform, the average being 73^. 



Wet-bulb Thermometers. — The relative humidity varied from 

 a comparative absence of vapor to complete saturation. Its mean 

 value was 0.463. During many days and nights, the degree of 

 moisture in the air was not one-tenth of what it could have held — 

 showing the dryness of the atmosphere — while at other times, but 

 very rarely, the mean of the day was 0.900. 



