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xi 
A. Guyot on the Appalachian Mountain System. 163 
always been applied in the calculations. A long familiarity with 
ay Lussac’s syphon barometer, with Bunten’s improvement, as 
well as with Fortin’s cistern barometer, modified by Ernst, has 
convineed me that the latter is to be preferred, notwithstanding 
its weight and its greater length, if the utmost accuracy compat- 
ible with the method is to be sought. The variations of capil- 
lary attraction, and the soiling of the tube of the short branch of 
the syphon by the oxyd of mercury in the Bunten barometer 
are serious inconveniences. The impossibility of repairing such 
barometers in case they are injured in travelling, is a still more 
serious difficulty. I carry with each of my Fortin barometers, 
two extra tubes and a bottle of purified mercury which enable - 
me in case of accident to reconstruct my barometer in two hours 
Acting in its behalf I superintended the construction of 
a series of meteorological instruments of which the manufacture - 
was entrusted to a skillful optician, Mr. James Green, of New 
York, and which are now extensively employed throughout the 
United States, under the name of the Smithsonian Meteorologi- 
cal Instruments, I especially endeavored to render these instra- 
ments strong, simple and adjustable. By the latter phrase, 
mean that their construction allows them to be regulated by a 
standard instrument so as to eliminate the error of zero in the — 
with that of the scale. In the barometer I obtain ‘this result 
¥ means of a moveable scale which slides upon the brass casing 
