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which pass through, impinge first on one plane, and arereflected to the other, they then pass out through thefront glass. By this optical arrangement, two suns arevisible to the eye of the observer moving in oppositedirections, and when they coincide, it is the instant ofapparent noon. The time can be ascertained by thisinstrument with considerable precision. The dipleido-scope allows of three observations of the sun: 1st, whenthe limbs touch; 2nd, when the images coincide; and3rd, when the limbs separate.
[The dipleidoscope was invented, a few years since, byG. M. Bloxem, Esq., and, when accurately fixed, the timeof apparent noon can be determined by it within one ortwo seconds. Two bright and sharp images of the sunare seen, which approach each other, and exactly coincideat apparent noon.—J. G.]
30. A dipleidoscope fitted up equatorially, which admitsof observations being taken from 9 a.m. to 3 p.m., and isrendered portable by having a magnetic needle. Whenthe instrument is set by the needle, the magnetic declina-tion of the place requires to be allowed for.
31. A superior astronomical clock, with a remontoiredead-beat escapement, invented by G. B. Airy , Esq.,Astronomer Royal (see paper in “Royal AstronomicalSociety’s Monthly Notices,” Nov. 11, 1842, by Mr. Airy .)
A large church clock. (Main Avenue.')
56 Drury, James, 16 North Avenue, North Street,Pentomille —Designer and Manufacturer.
Clock dial-case in brass, adapted for clocks in hotclimates, at sea, and in bakers’ shops, as they are notaffected by steam and heat.
57 Frodsham, Charles , 84 Strand— ChronometerMaker.
1. Astronomical clock, with mercurial pendulum, andGraham’s dead-beat escapement.
[Mercurial pendulums, and Graham’s dead-beat escape-ment, are now used in nearly all astronomical clocks.The number of such clocks now in London exceeds 200;about 60 of these, chiefly the property of eminent chro-nometer makers, are rated on Greenwich mean time, and
it is found that their performance is such as to adapthem for astronomical uses. They certainly reflect g* e ^credit on the various artists engaged in the sciencehorology.
The mercurial pendulum is so called from the circuit*stance of the bob being of mercury enclosed in a cylinderscrewed to the bottom of a steel rod. Gridiron pen**lums were in use before mercurial pendulums weie
adopted: they consisted of an assemblage of brass a*steel rods so arranged that, owing to the difference in ***expansion of brass and steel, the brass rods carried 1 ^bob up, while the steel ones let it down. The shapependulum bobs is important. Until lately they werethe form of a double convex lens, this form offering h®little resistance on passing through the air, when its eag
was always in the plane of motion; but, as it was
liable
to be a little twisted, a varying resistance was °d ere ^J.and which is not the case in the adoption of the for 111the cylinder, which probably is the best shape for the bo -lt is usual to call those clocks astronomical which a*°used in observatories for determining the right ascensionof the heavenly bodies, and they are such, includesevery appendage which contributes to accuracy * nmeasurement of time under all the changes of atmosphc
temperature.— J. G.] ^
2. Marine chronometers, on a new calibre, with A rI |”and Earnshaw’s detached escapement; the compensa ^balance is of the ordinary kind, with Arnold’s b at o0auxiliary compensation. This new calibre is based **P,the plan of the diameter of barrel, fuzee-wheel, ®extreme diameter of the balance being the same,one inch and five-tenths. The total weight ofcompensation balance is S dwts., being as the p ont< L,.of the barrel. Thus if a barrel, one inch in diaro e ^by three-tenths of an inch in depth, will carry a bals„ n( jweighing 20 grains, a barrel of the same diameter,of double the depth, will carry a balance wel ^ n t j l e40 grains. The balance-spring is 15 inches long ! jdiameter -jjfj, the thickness of wire -|J§ 5 by broad, jthe number of turns 10 to 12. The wheels (escape-)'’included) are each five times the diameter of the** ^spective pinions—that is, the pinion upon which
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Frodsham's new Calibre for Watches.