HISTORY OF PHYSICAL ASTRONOMY.

125

mass of Venus. In 1844, Le Verrier instituted a profound examinationinto tlie theory of Mercury . He calculated all the terms of perturbationwhich could sensibly affect the motion of the planet, and then employedthem in the formation of preliminary tables, for the purpose of obtainingcorrections of the elements. His researches were based on about 400meridional observations of the planet, made at the Royal Observatory ofParis during the present century, and also on a considerable number oftransits, all of which he submitted to a careful discussion before intro-ducing them into the equations of condition. Having in this manner ob-tained new elements of the planet, and also a correction for the mass ofVenus , he constructed tables, which were found to represent the observa-tions with wonderful precision. The following interesting account of atransit which had been previously calculated by means of them is givenby Mr. Mitchell, of the Observatory of Cincinnati, in the United Slates:—“ Five minutes before the computed time of the contact, I took myplace at the instrument; the beautiful machinery that carries the telescopewith the sun was set in motion, and the instrument directed to that partof the sun’s disk at which it was anticipated the contact would take place.And there I sat, with feelings which no one in this audience can realise. It w'as my first effort; all had been done by myself. After remain-ing there for what seemed to be long hours, I inquired of my assistanthow much longer I would have to wait; I was answered four minutes. Ikept my place for what seemed an age, and again inquired as before ; hetold me that but one minute had rolled by. It seemed as if time had foldedhis wings, so slowly did the moments crawl on. I watched on until Iwas told that but one minute remained, and within sixteen seconds of thetime I had the almost bewildering gratification of seeing the planet breakthe contact, and slowly move on till it buried itself, round, and deep, andsharp in the sun.”

The foregoing account may serve to give the general reader an idea ofthe accordance existing in the science of astronomy, between theory andobservation, even under circumstances of the most disadvantageous cha-racter. In the present case, this accordance reflected the highest honouron the accuracy of Le Vender's researches, and augured favourably forthe future efforts of that illustrious geometer.

As Mercury disturbs the motions of the other planets only in a verysmall degree, and as it is moreover unaccompanied by one or more satel-lites, it has been found very difficult to ascertain the precise value of itsmass. In the absence of more satisfactory methods, this element hasbeen determined by assuming the densities of the planets to vary in theinverse ratio of their mean distances from the sun, and then combiningthe density of the planet found in this manner with its volume, as indi-cated by its apparent diameter. By this means astronomers have ob-tained T.TiArniF f° r va l ue of the mass of the planet, the sun’s massbeing represented by unity. Encke, however, has concluded from his re-searches on the perturbations of the comet which bears his name, that themass of Mercury does not exceed -j.aB-J-jTT *• Le Verrier , on the otherhand, assigns as the most probable value of the mass. Mr.

Rothman, again, by comparing the theory of the planet with the ob-served motion of Venus ’s perihelion, estimates the mass at -j.tiplwtt t-This result agrees very nearly with Le Vender's, but it must be admitted

* Pro. Astr. Soc., February, 1842.

t Mem. Astr. Soc., vol. xii.