270 Rev. W. Hodgson’s Description of the Chronoscope. 
support (by means of a screw bearing upon one of its axes,) 
in such a position that all the edges of the prisms are parallel 
to the horizon (an adjustment which is readily made by ob- 
servations on a plumb-line) and so that the plane, passing 
through the faces of the prisms which are in contact, is in- 
clined to the horizon at any angle which is a few degrees 
less than the sun’s meridian altitude, then, by moving the 
instrument upon that axis which is not clamped, two images 
of the sun may be seen, by looking towards that body through 
either of the prisms, to approach, coincide, and separate, at 
certain instants before noon ; and again, at corresponding in- 
stants after noon, to approach, coincide, and separate, as be- 
fore. By observing the times at which these phenomena 
occur, and taking the semi-sum of the intervals between the 
corresponding pairs of observations, or a mean deduced from 
the whole six observations (correcting, if necessary, for the 
change of declination) the ¢rwe apparent noon is at once de- 
termined. By repeating this process on successive days, the 
rate of the watch may be ascertained and corrected. 
When the time is thus found, the instrument, at any suc- 
ceeding noon, may be permanently fixed with the plane pass- 
ing through those faces of the prisms which are in contact, 
perpendicular to the horizon, and with the two solar images 
in accurate coincidence. In this position, the instrument 
affords the means of observing with accuracy the passage of 
the sun across the plane of the meridian on any future occa- 
sion when that luminary is visible at the time. The passage 
of the moon also across the meridian may be similarly ob- 
served, and from thence the longitude of the place, if un- 
known, may be computed. 
If the time of the true apparent noon is supposed to be 
known, and the instrument is required only to keep the true 
time deduced from some other source, the two axes above 
mentioned may both be dispensed with, and it will be suffi- 
cient to place the edges of the prisms so as to be perpendi- 
cular to the horizon, and to have the two solar images seen — 
in exact coincidence at the instant of true apparent noon. 
If the prisms, instead of being placed in close contact, are’ 
separated by a narrow strip of thin sheet-metal placed be- 
