Annual Report on the U.S. Coast Survey. 309 
Measurement of a new Base Line.—During the past year the 
measurement of a new base line nearly seven miles long, has 
been made by the superintendent in person, after a reconnoissance 
by F. H. Gerdes on the Gulf of Mexico. The principal novelty 
by in Great Britain, and b r. Borden in this country; but 
the method of obtaining the compensation was entirely different 
from that used by these gentlemen, and served to remove a cause 
of error which has been strangely overlooked in the different ap- 
plications of the compensating arrangement as usually employed. 
bar of brass and of iron of the same dimensions and exposed 
to the same source of heat, will not arrive at the same tempera~ 
ture in the same time; the rapidity of change will depend upon 
the difference of conducting power, the difference of specific 
lengths. To remedy this defect, the surfaces of the different 
produced. - 'This apparatus was constructed in 1845, and first used 
with microscopes in the autumn of that year, and is briefly re- 
tr. Saxton’s reflecting pyrometer was employed, these changes 
e became very perceptible, and it was necessary to resort to direct 
J ®Xperiment upon the material of the bars themselves to obtain 
: ven approximate compensation, and there to correct a small resi- 
: dual quantity by applying a covering more absorbent of heat to 
e bar than the other. If such changes have not been perceiv- 
; ed hitherto, it has been because inadequate means were used to 
: detect them. I am indebted to Lieutenant A. A. Humphreys 
and to Mr. Joseph Saxton for the perseverance and skill with 
Which the experiments necessary to a complete adjustment of 
the apparatus were made.” 
