HISTORY OF FIIYSICAL ASTRONOMY.

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increase of optical power, new accessions will be constantly made to thevisible extent of the universe.

The results which Lord Eosse has already achieved by means of hispowerful telescopes, clearly point out the course which must be pursuedin order to acquire some degree of knowledge respecting the nature andconstitution of those magnificent systems of sidereal bodies which havebeen found to pervade the immensity of space. In the first place, it ismanifest that a careful delineation of the different parts of each systemforms an indispensable preliminary to any progress in this branch ofenquiry. It is only by a subsequent comparison, instituted betweennebulae of every observable variety of form, that any knowledge can beobtained respecting the real structure of those wonderful systems.Among them there are, doubtless, systems of different orders, as we findexemplified to a limited extent in the solar system, but among those ofthe same order we have just grounds for suspecting that a kindred formof structure prevails. It is probable that such systems are viewed inevery variety of position relative to the earth, and lienee we may reason-ably expect that, by an extensive collation of their apparent forms, it willbe possible to arrive at some definite conclusions respecting their actualconfiguration in space. Formidable as the problem relative to the struc-ture of these systems may appear when considered from this point of view,its difficulties are vastly enhanced by the internal changes which are con-stantly taking place in each system. It must be borne in mind that,although the various bodies of which each system is composed are doubt-less linked together by intimate physical relations, the condition whichthus subsists is one of dynamical stability rather than statical equilibrium.In fact, we are amply justified, both by reason and analogy, in supposingthat the constituent bodies of every system are revolving in curvilinearorbits under the influence of their mutual gravitation. It is manifestthat from this cause each system will be liable, in process of time, toundergo a modification of form. Hence it follows, that not only willsystems of the same order exhibit different external forms when com-pared with each other, but even the same system will vary from timeto time relatively to itself. That the eccentric position of the observer,and the displacements arising from the internal movements of its con-stituent bodies, may cause a system to appear very complicated, while atthe same time its various parts are adjusted in the most perfect harmony,is evident, from a consideration not only of the planetary system itself, butalso of those miniature systems of which it is mainly composed. Whatmore admirable arrangements can the imagination conceive than thosewhich pervade the system of Jupiter 's satellites, and yet when this exqui-site mechanism is casually viewed through a telescope, what can be moredevoid of symmetry than the configurations of its different parts ?

It is manifest, from the foregoing considerations, that, in order to acquirejust ideas respecting the construction of those vast systems of worlds whichthe telescope has shown to be distributed throughout the immensity ofspace, an assiduous course of observation and research, prosecuted through-out a long succession of ages, is absolutely indispensable. The investi-gation of the dynamical laws by' which those systems are governed andtheir stability is assured, must be regarded as an ulterior object. It isonly in the case of the more simple systems, the relative movements ofwhose constituent bodies admit of being readily deduced from observation,that the dynamical conditions of the system can form a feasible subject