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 observations of the transit of Venus in the year 1639, and of the distances of all planets from the Sun as deduced from the observations of the transit in the year 1761 / by James Ferguson .