LET. XVIII. OF THE SUN, MOON, AND PLANETS. jJOI

therefore his distance will be determined to with-in its five hundredth part at least, if the paral-lax be not found less than we have supposed;for forty times twelve and an half is fivehundred.”

The Doctor having pursued his subject thusfar, in this popular and easy way, proceeds toillustrate it by a figure; but as he has intro-duced several things into his calculation whichcould not be understood by a person unacquaint-ed with mathematical principles, I shall endea-vour, by means of a simple unimbarraffedscheme, to give you such an idea of the matter,as will at once convince you of the practicabi-lity and certainty of this method. The two lasttransits were in the years 1761 and 1769, and asthere will not be another before the year 1874,when most of the human race, now living, willin all probability be dead, it will be unnecessaryto trouble you with a long account of the me-thods made use of for observing these eclipseswith accuracy and precision, or to enter into anyother particulars, than what are sufficient forour present purpose.

The diameter of the earth is no more than apoint in comparison to the distance of the fun ;and therefore if the fun were viewed at thefame instant, by two observers, on opposite sidesof the earth, his center would appear to both ofthem to be exactly in the fame point of the

heavens.