Astronomy Explained Upon Sir Isaac Newton's Principles And Made Easy to Those who Have Not Studied Mathematics : to which are Added, a Plain Method of Finding the Distances of All the Planets from the Sun, by the Transit of Venus Over the Sun's Disc, in the Year 1761 : an Account of Mr. Horrox's Observation of the Transit of Venus in the Year 1639 : And, of the Distances of All the Planets from the Sun, as Deduced from Observations of the Transit in the Year 1761