Ackenuan.] ^^^ [July 15, 



The instruments have no frame, and are freely suspended without be- 

 ing sliaken by the wind. Tlie absolute minimum is given by a Ruther- 

 ford spirit therm., and the absolute maximum by Negretti &. Zambra's 

 mercurial therm., both Salleron's construction at Paris. 



The hourly observations of temperature are performed by "Breguet's 

 therraometrographe horaire No. 6," [See Ai'ago, not. scientif. vol. V, 

 pp. 628-632, and Desains: physique, vol. I, page 247; or Daguin: phys. 

 vol. II, page 546, etc. ] This instrument having an arbitrary scale, it was 

 compared with a standard therm, in two constant temperatures, and fur- 

 ther checked by numerous simultaneous obsei'vations. Breguet's No. 6 

 acts in the most satisfactory manner, but is much more sensible than 

 other thermometers, so that for the comparisons the instruments were 

 read at a distance with the aid of a cathetometer, and further all the cares 

 taken to obtain correctness, etc. 



The exposure of all the instruments is as follows: 



A square room of 14 feet a side, has openings towards the four cardinal 

 jDOints, a covered gallery on the South side, is without ceiling, covered 

 with shingles, so that the air circulates freely day or night, from what- 

 ever direction the wind is blowing. 



On the North side is the window furnished with latticed blinds, painted 

 white, nearly of the same form as prescribed in the "Directions for 

 meteorol. observ." Smithsonian Institution 1860, fig. 2. Elevation of 

 tliermometers above ground 10 feet. 



The mean daily temperature (and consequently the mean monthly and 

 annual) are the results of the 24 registered hourly observations. The 

 given factor is the co-efficient by which the difference between the abso- 

 lute maximum and minimum is to be multiplied, and the product added 

 to the minimum, in order to obtain the same mean daily temperature as 

 given by Breguet's hourly thermometrograph. 



About ten personal observ. were made daily, with free thermom., 

 psychrom., barometer, winds, clouds, &c., &c., besides the reading of the 

 maxim and minim and the said thermometrograph, thermometer exposed 

 to the sun, to nocturnal radiation, etc. 



The second decimal of Fahrenheit degrees does not occur in obsei'va- 

 tion, and is either the result of the mean addition or produced by the re- 

 duction of Centigrades into Fahrenheit degrees. 



The barometers, Fortin's, had been compared with the barometer at 



the astron. observatory in Paris, and the makers had given the correction, 



a constant, for every one. Further, the observations were corrected for 



capillarity, the column reduced to the temperature of zero degree (33° 



Fahrenheit) and reduced to the mean level of the sea by the formula of 



Jamin, Cours de physique de I'ecole polytechnique, vol. I, end of page 



363. 



2fT + m H 



log 



X = 18405™ (1 + 0.002552. cos 2 L) fi + -I-tHl 



L 1000 J 



h 



H & h being reduced to 0° C. X being known the value of H gave the 

 pressure on the level of the sea. 



