Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: they have passed near, while they themselves have been diverted from their course: indications that their mass is small, and, as their bulk is considerable, that they are probably in the aerial state. Also, they present no phases, which seems to show that the sun’s light penetrates their substance, and that light is reflected from every part of it. Q. Have the periods of time which any of the comets require fortheir revolution round the sun been calculated ? A. Yes. The periods of H Alley’s comet, the comet of Encke, and the comet of Biela, have been com. puted, and the correctness of the calculation proved by their return several times. A very bright comet, seen by the celebrated astronomer Halley, in 1682, was calculated by him to be the same which had appeared in 1531 and 1607,?at intervals of between seventy-five and seventy six years. He accordingly predicted its return about 1759, when it did appear, nearly about the calculated time; and it has again returned at the computed period, in 1835. The other two comets whose periods have been ascertained return in short periods: that of Biela in 6f years; that of Encke in 3 J years. These two comets are not visible to the naked eye. Q. Has not the comet of Encke led to some singular speculations regarding the existence of a fluid spread out through space ? A. Yes. Its period of revolution round the sun is found to be diminishing. This is attributed to the resistance opposed to it in its progress by some material fluid through which it passes,?which weakens its centrifugal force, gives the sun’s attractive force larger proportionate power, enables that body to draw it nearer, ad therefore into a smaller orbit, through which it performs its course in a shorter time. INFLUENCE OF THE ATMOSPHERE ON ASTRONOMICAL APP…
