370 



working days, not reckoning the time lost by bad weather, or oc- 

 cupied by change of camp, and by comparisons of the apparatus. 

 The length of the apparatus was compared, before and after the final 

 measurement, with a standard iron bar with which it had been com- 

 pared in the Coast Survey office, by using Mr. Saxton's reflecting 

 pyrometer. 



The accuracy with which a remeasurement of considerable length 

 could be made was tested more than once in the measurement, but it 

 was determined to make a more complete direct trial by establishing 

 intermediate marks, and noting by the microtelescopes attached to 

 the ends of the bars the deviation at intervals. The greatest devia- 

 tion in the length of seventeen tubes was between one and two-hun- 

 dredths of an inch, the average in cases of repetition, not regarding 

 signs, was five one-thousandths of an inch, the final error at the end 

 of the one-third of a mile remeasured was nothing. The probable 

 error of remeasuring one hundred and twelve yards was less than 

 five ten-thousandths of an inch, making on the whole length of the 

 base, and supposing all the errors to fall in the same direction, which 

 is physically most improbable, less than nine-tenths of an inch. The 

 great practical difficulty found at the outset was to obtain a mark 

 which would stand unmoved in the sand to which to refer the ap- 

 paratus on recommencing a measurement; this was satisfactorily 

 obviated after many experiments, and the marks which 1 have just 

 stated to have been placed at intervals may be assumed generally to 

 have been stationary from one measurement to another. Incidentally, 

 this remeasurement gave a strong test of the perfect compensation of 

 the apparatus under sudden changes of temperature, as well as for 

 different stationary temperatures. A storm came up after the second 

 measurement was commenced, which interrupted it for between one 

 and two hours, and cooled the air suddenly about four degrees. The 

 second measurement was therefore made at a lower temperature by 

 some degrees than the first, and under exposure to a sudden fall of 

 temperature. 



The chair having again been taken by Prof. Bache, — 

 Professor Henry made a communication relative to some ob- 

 servations on the Aurora Borealis, with the object of determin- 

 ing the height of the meteor. The result of the observations 

 tended to establish the fact, that the arch of the aurora, like the 

 rainbow, is a local phenomenon, each observer seeing a differ- 

 ent object. 



