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It yet remains to be fhewed, how the annual Mo- 
tion of the Earth in her Orbit, caiifes the Change of 
the Sun’s Declination : This cannot be done by the 
Globe limply taken, but is very well fhewed by the 
Inftruments called Orreries : But to thefe their Coftli- 
nefs is an Obje<ftion, not mentioning others from a 
want of due Proportion in the things they exhibit. 
I had therefore an Inftrument made, which conlifted 
only of a round Trencher of Wood, a Circle of 
Brafs upon the Face of it, and between thefe three 
Wheels of the fame Dimenfions and Number of 
Teeth: The innermoft Wheel was fixed to the 
Wood in the Centre, the third had its Axis come 
through the brafs Plate, round which was a brafs 
Circle having a Socket making an Angle with it of 
66 \ Degrees j in this Socket was fixed the Axis of a 
little Globe, having an Horizon about it, to repre- 
fent the Circle feparating Light from Darknefs, the 
Sun being fuppofed to be in the Middle of the In- 
ftrument. While the brafs Plate is turned round 
through the Scale of Months and Days exprefl'ed on 
the under Plate, the Axis of the Terrella is kept ail 
the while parallel to itfelf, by means of the fecond 
Wheel placed between the two above-mentioned, 
and fo the Change of the Sun’s Declination, or rather, 
which comes to the fame Purpofe, the different Po- 
fition of the Equatorial Axis with refpedt to the 
Circle feparating Light and Darknefs, is exhibited all 
the while the Earth is going round in her Orbit, By 
placing the Axis of an ivory Ball having one half 
blacked, upright in the middle of the Circle which 
carries the Terrella^ this little Infirument will ferve 
to explain the ^hienomem of the Moon’s Thafes, 
T t Having 
