47° 



Observed Orbital Curve. 



the object being still, and on the duration of the attacking series of chemical 

 rays or vibrations on the exposed surface in the photographic progress. The 

 sensible sun's heat rays may be in proportion to the time each heat ray or vibra- 

 tion reaches the earth or is in action,, as well as to the distance. This may help 

 to explain why the sun's heat is least when the earth is nearest to the sun, for the 

 velocity of the earth is then greater : the angle of incidence of rays also varying. 

 Fig. 36 represents an ellipse or orbit with the large attractive body at the 

 point F, one of the foci of this ellipse. M' M" are smaller bodies supposed to 

 be deflected, from the straight line along which they were projected, by the force 

 of the great body at F. It will be seen by reference to Fig. 37, that the orbit 

 cannot be a true ellipse which any body M f or M' A will follow. The reason is 

 that the cm-ve in the ellipse is really greatest at A, while it approaches a straight 

 line at D. Attraction from a single force, F to M, could not produce such a 

 result, which would only occur if there were another attracting body of equal 

 size at the other focus F ; that is, an ellipse is a two-centred curve and has 

 none of the properties of the single-centred orbital curve Fig. 37. 



Orbital curiae not em ETZipVB* 



ac. 37, 



M tcM tracks of Mo on enteraiy Oxbft. 



A.TYLOlt.DEL. 



Fig. 37 is an orbital curve in which the earth moves round the sun supposed 

 to be stationary, or in which the moon moves round the earth when that body 

 is supposed to be stationary for the purpose. The deflection of the small body 

 M, M, M, &c. (from a straight line), projected into the orbit, is in proportion 

 to the weight and nearness of the small attracted body M, M, &c, to the great 

 attracting body at E. The orbital curve at A is greatest because the force E is 

 nearest to that part of the curve, and therefore E deflects M (extending its 

 orbit and continuing it in the track) most from a straight line. In Fig. 37 the 

 attractive force from E is at its mean at C and D, and therefore the curve at 

 those points is at its mean at C and D. The gradient or slope of the orbital 

 curve is at its minimum at B, because E can deflect the small body M 4 least 

 at B from a straight line. The gradient or curve is greatest at A, because the 

 attractive force at E can deflect the small body M the most from the straight line 

 in which a body like the moon or earth was originally projected through space. 



