1. _What amount of matter precipitated upon the sun would develop the quantity of heat actually emitted?_--This question has been satisfactorily discussed by eminent physicists, and it will be sufficient for our purpose to give the result. According to Professor William Thomson, of Glasgow, the present rate of emission would be kept up by a meteoric deposit which would form an annual stratum 60 feet in thickness over the sun's surface.
2. _Could such an increase of the sun's magnitude be detected by micrometrical measurement?_--This inquiry is readily answered in the negative. The apparent diameter would be augmented only one second in 17,600 years.
3. _Is there any known or visible source from which this amount of meteoric matter may be supplied?_--Thomson, Mayer, and other distinguished writers regard the zodiacal light as the source of such meteorites. The inner portions of this immense "tornado" must be resisted in their motions by the solar atmosphere, and hence precipitated upon the sun's surface.
4. _Would this increase of the sun's mass derange the motions of the solar system?_--To this question Prof. Ennis gives an affirmative answer; his first objection to the theory under consideration being stated as follows: "The constant accumulation of such materials, during hundreds of millions of years, would increase the body of the sun and its consequent gravity so greatly as to derange the entire solar system, by destroying the balance between the centripetal and centrifugal forces now acting on the planets."[24] This, it must be confessed, would be a valid objection, if the meteoric matter were supposed to be derived from the extra-planetary spaces. As their source, however,--the zodiacal light--is interior to the earth's orbit, it can have no application to any planet exterior to Venus. Most probably the greater portion of the meteoric mass is even within the orbit of Mercury, so that the effect of its convergence could scarcely be noticed even in the motion of the interior planets. In pre-historic time the zodiacal light may have extended far beyond the earth's orbit. If so, its convergence to its present dimensions was undoubtedly attended by an acceleration of the earth's mean motion. We can of course have no evidence that such a shortening of the year has never occurred.