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Fig. 1..

The Centre Figure represents the An
nual Revolution of the Earth round
the Sun on its Orbit, which is an
Ellipse, the excentricity of which is
about 583,691 leagues and on the
first of each Month. The gradua
ted Circle which at a small dis-
tance surrounds the North Pole
in each figure of the Earth,
shows the Parallel of Great Bri
tain, this Circle is divided into
24 hours.

The figure to the right marks the
hour of Sun-rise, that to the left
the hour of Sun set. The point

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indicates the position of Great Britain with reference to the Sun at the various hours of the Day and Night -for Example. it is 2 o'clock AM on the first of June, and the horary Circle shows us that at this time the hour of Sun-rise is 4 3" and that of Sun-set 7" 51" the Night therefore will continue 2 3" Ionger. Again on the first of Decem ber the Sun rises at 7" 40" and sets as the horary Circle indicates at 4 4 the time supposed being 1" 40" the day will continue 2 24" longer. At this period of the Year the Earth approaches the Winter Solstice when its North Pole is enveloped in darkness for two months will have reached the middle of its long night.

Fig. 2.

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Suppose a moving body M. subjected to the in-
fluence of two forces. One in the direction In S.
the other in the direction In.. the intensity being
indicated by the right lines I. P. and I. V, the mo
ving body can neither go towards Snor, but must take
an intermediate Course I. O, with a Velocity represented
by M. It is thus clear that the moving body II. will
take the same direction and have the same Velocity as one
force represented in Magnitude by M. and in direction by I.
0, would have imparted to it. The name Resultant or equivalent
force is given to this When a Curve is described by Virtue of a force
directed to a fixed point S, it may be resolved into two-One in the direc
tion of the Radius m S, the other of the Curve A This accelerating force is
supposed to act only at the beginning of each moment while the Motion is uni
form the Vector Radius In S, then will trace a small triangle MSA If this force
ceased to act the following moments the Vector Radius would trace a fresh triangle
equal to the first But at the beginning of the next moment the accelerating force I k.com

bines with IB,and causes the moving body to exhibit the effects of the resultant force A In the
sides of which Ap A B, represent these forces.

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EPTEMBER

JB LIPPINC

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THEORY OF THE TWILIGHT

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For some time after Sun-set the raysS still illumine the Atmosphere, diffu sing themselves through space, and Night comes on almost insensibly. this is called twilight, and the dawn is a similar Phenomenon, which precedes Sun-rise. The Solar ray S. O. which enters the Atmos phere at 0. instead of coming out at M. will be turned aside, refracted or bent more and more as it penetrates the various Strata of Air, and will at last reach the Earth describing the Curve O. G. The observer at G. would thus see the Sun much longer at E. than he really was visible above the horizon. The effect of the dawn is the reverse of this let the Earth be represented by A. B. C. D. and G. where the Sun rises. the horizon of H. G. R. receives light from all parts of the Heavens either directly or by reflection, the point A. at which the sun has not actually risen, has above its horizon the lenticular portion of the Atmosphere K. L. R. indirectly illuminated, the dawn is most brilhant at K. the tint becoming more feeble until it reaches R. its utmost limit above the horizon, at B. it is Midnight, and at D where the Sun is Rising the whole of the horizon is illu minated, while at C. the lenticular portions of the Heavens T. N. X. only receive light. The extent of twilight is shown on the figure by a Zone of lighter shade than that which marks Night. Suppose M. the Meridian of London and it is Noon at G. (90° West Long.) it is 6 A. M. and at Y. (45" West Long) 9 o'clock, by the effect of Atmospheric attraction the Sun would appear to be ali J. at Q. (45° East Long.) the Sun would appear at E. at 3 o'clock P. M. Thus as the Earth, the North Pole being at P. turns on its Axis from West to East, as indicated by the Arrows, the Sun will appear to occupy the opposite points F. J. S. E. U. This figure is drawn at the time when the Sun appears to describe the Equator

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