10 



made by James Green, of New York, especially for this work, and were the 

 best instruments he could produce. The two mercurial barometers had double 

 verniers reading to -002 of an inch, and were capable of recording 18,000 feet 

 elevation. They were each packed first in a wooden case, which was then 

 placed in a leathern sheath, and thus thoroughly prepared for rough usage 

 and mountain work. They worked admirably together, as was shown by a 

 long series of comparisons before and after leaving Princeton. A comparison 

 was also made before and after each day's work, and the extreme variation was 

 never found to exceed -004 of an inch. We carried with these instruments a 

 case of repairing articles, including bottles of mercury, leather, thread, etc.. 

 also six extra tubes in a box, so that in case of an accident we would not be 

 left hopelessly in the lurch. But such was the quality of the workmanship dis- 

 played in these instruments that notwithstanding the exceedingly rough usage 

 they received, not a repair was necessary, and they were brought home in 

 as good working condition as when they started out. Of the two aneroids, one 

 read on its face to 20,000 feet, with 5o-feet divisions. It was also divided to 

 14 inches, again divided to hundredths. The other read to 15,000 feet, with 

 5O-feet divisions, and was also divided to 14 inches, and hundredths. These 

 two instruments were selected from six first-class ones as the best of the lot. 

 They were tested by trials under the air-pump in the physical laboratory at 

 Princeton, and worked well, though they have not done so well since their 

 return. 



LEVELLING-ROD. The levelling-rod was made by Keuffel & Esser, of New 

 York. It was an engineer's telescopic rod, and read to thousandths of a foot. 



LEVELS. The two levels were the ordinary pocket-levels, with level-tube 

 on the inside of the tube, and an inclined mirror at the bottom of the same to 

 reflect the image of the bubble. They were made by Mr. Green, of New York. 



PRISMATIC COMPASS. This instrument was also made by Mr. Green, and 

 of the same pattern as is used by the United States Engineer Corps. 



COMPASSES AND CLINOMETER. These were made by Mr. Green. There were 

 three small compasses for pocket use in brass cases. The clinometer was a 

 3-inch compass, mounted upon a square piece of brass, one edge of which 

 served as a base for the clinometer index. This compass was also provided 

 with sighting slits. 



TELESCOPES. These were made by Bardou fils & Cie., of Paris, having 

 glasses of i^ and 2 inches diameter. Both of them were pieces of fine work- 

 manship, and were of great service on account of their clear definition and high 

 powers. 



THERMOMETERS. These were graded according to Fahrenheit and made by 

 Mr. Green. Each had a wooden case of its own. One of them was used for a 

 wet bulb thermometer in meteorological observations. 



