ACADEMY OF SCIENCES. 9 
hydrography no less necessary than this skill and dexterity, is rapidity of 
execution in determining positions; and, to this end, with two observers, the 
requisite promptness and oneness of action are found deplorably deficient, 
and that, too, at moments the most critical. A sunken rock or reef is to be 
determined, and on it a sounding gotten. The rock is found, the ‘‘ cast’ is 
taken—the word ‘‘ stand by for an angle ’’ is given—and at length comes the 
response, r-e-a-d-y; by which time perhaps a tangent screw is jammed (hard 
up) or the boat has drifted from over the rock, and thus the reward, for hours, 
or it may be for days, of persistent and arduous exertions is lost. And such 
mishaps must ever continue to recur where two observers are called upon to 
act quickly and simultaneously under exciting circumstances. 
But if in the boat there is only one observer, with a Protracting Sextant, then 
we may confidently expect that promptness and oneness of action, in 
observing, under every contingency, which are so essential to the rapid and 
successful execution of a hydrographic survey. 
Although we do not presume to say that the theory of this instrument is 
so obvious, or its manipulations so simple, that ‘‘the simpleton, though he 
run may understand,” or that the smatterer and blind routiner (who could 
not look a quadrilateral in the face without blushing) may manipulate it with 
ease and accuracy, yet we do not assert without the fear of a contradiction, 
that to the eye of the ingenious geometer, its theory is most clear, and that 
in the hands of the hydrographer, who isa master of his profession, this 
Protracting Sextant will be tound the ready and efficient means of determining 
and plotting (unassisted and alone) his position, with a facility, ease and 
accuracy not now attained with two ordinary sextants and one protractor in 
the hands of two observers and one plotter. 
The Secretary read the following from Professor George 
Davidson: 
Transit of Venus. 
BY GEORGE DAVIDSON. 
To the California Academy of Sciences, San Francisco, Cal.:—The instructions 
of the Commission permit me to give general results of our work, and I con- 
dense as much as possible for presentation to the Academy, our labors of 
preparation and final results. 
We have determined the difference of longitude by cable, between Nagasaki 
and Vladivostok, whence it will be carried westward to St. Petersburg by tel- 
-egraph, and in connecting the Venus Station with the Telegraph Observatory 
we have determined the latitude and longitude of the French Venus Station 
and two other points on the bay. 
We have determined the latitude of our station by the Talcatt method; ob- 
serving upon twenty pairs of stars for five nights. 
We have observed fourteen occultations of stars by the moon for longitude 
differences with Peking and other stations. This was work which we had to 
