Modern Astronomers have made use of the swiftest Velocity of a Cannon-Ball as continued thro' the Space they would so describe, and in this Light, the Distance to the Sun has been by many compar'd to twenty-five Years Motion of a Cannon-Ball, supposing it to travel at the Rate of 100 Fathom in a Moment, _i. e._ _the Pulse of an Artery_; and that a Journey so performed to one of the nearest fix'd Stars, would take the same Body at least 100,000 Years before it could arrive there. But the Method I have chose to convey my Ideas of the Magnitude of the planetary Bodies, and the Extent of the visible Creation to you, I am willing to hope you will find still more familiar, comprehensive, and easy: And it only depends upon your Remembrance of a very few known Objects, and their neighbouring Distances, which may be presumed you are, or have been, very well acquainted with. You have not only very lately but very often been in _London_, and must, I think, retain some Idea of the Dome of St. _Paul's_, tho' I own I ought not to be sorry if you should chance to have forgot it, provided it might prove a Means of making your Visits more frequent. The Diameter of the Dome of this Church is 145 Feet: Now if you can imagine this to represent the Surface of the Sun, a spherical Body 18 Inches diameter, will justly represent the Earth in like Proportion; and another of only five Inches diameter, will represent the Moon. The Truths of these Proportions I have shewn in my _Clavis Cœlestis_; and the Reason why I have here fixt upon the Dome of this Church for my first Object of Comparison, will naturally appear from what follows. From the Magnitude of the Earth on which we live, as from a known Scale with respect to its Parts compar'd with our own Bodies, we naturally frame our first Ideas of Extent, and fix our Rationale of Remoteness; by which we are sufficiently enabled to judge of all other sensible Distances within one finite View. And hence by the undoubted Principles of Geometry, having first given the Measurement of the Earth in any known Proportion with any other Quantity most familiar to our Senses, and the Angle of Appearance, or Parallax to any perceivable Object, we can easily find in homogenial Parts its true Distance from the Eye. And thus allowing for some small tho' unavoidable Errors, that may possibly arise from the Difficulties of Observation (especially small Angles and minute Quantities) we can always determine to a sufficient, and very frequently to a just Exactness, the relative Distance of all visible Bodies, remote or near, such as the Planets, Comets, and the Sun. [AS]In this Manner Astronomers having procur'd a comparative Standard, reduc'd to some known Measure, as _English_ Miles, Leagues, Semi-Orbs or Orbits, with all the Force of analogical Reasoning, clearly can demonstrate the Place and Distance of any Object within the Reach of Observation, and judge of Distances almost indefinite.