3OO OF THE DISTANCES AND MAGNITUDES

tions, and diligently apply themselves with alltheir might to the making of the necessary obser-vations ; in which I earnestly wish them all ima-ginable success: in the first place, that they maynot, by the unseasonable obscurity of a cloudedsky, be deprived of this most desirable sight; andthen, that having ascertained with more exactnessthe magnitude of the planetary orbits, it mayredound to their immortal fame and glory.”

" And thus have I shewn, fays the Doctor,that, by this method, the fun’s distance may bedetermined to within its five hundredth part,which will doubtless appear very extraordinaryto some. But if an accurate observation be madeat each of the places above-mentioned, I havealready demonstrated that the durations of theeclipse made by Venus, will differ from eachother by seventeen minutes of time ; that is,upon a supposition that the sun’s parallax istwelve seconds and an half. But if the differ-ence should be found, by observation, to begreater or less, the fun’s parallax will be greateror less in nearly the fame proportion. And sinceseventeen minutes of time are answerable totwelve seconds and an half of solar parallax, forevery second of parallax there will arise a dif-ference of more than eighty seconds of time;so that if we have this difference true to twoseconds, it will be certain what the fun’s parallaxis, to within a fortieth part of a second; and

therefore