llow is tlic lioat ui tht' mui niaintaioed ] low long lias it Itisted alresdy ? I low long willit continuo? Are heri wiy signwof increaie oroi'diinimilion? After llfiriuinjr that, in the present statu ui' soieiioO, only somewbat vague and unsatisfaetory replio. are posible. Trof. ïoun; holds that. m lar aa observado goes, w can only say that the outpouring ot tlio BOUI beat, amaung as it is, appcars to have cono on UDChangcd tlirougli all the oenturiw ot' human lustory. The autbor thinks tht tbeie 8 spme tretli in each of the two theonos wliich have ,,.,.,, propesed w account lor tbc sim a lire. As to the firet, the impact of nu matter, it is iuite certain thai solar heat is i hn prodaoea : lut the question is whtthcr ihe supply "f meteoric matter is ufficient to account f'or any real prppOTÜon of the whole. Alter giving Sir Wilham Jhonipson'a ealculation of the omount ol heat whiob would be irodiuc(l L.y each of the plunotH falliug ntó the sun iroui lts present orbit, by wliieh it appears that Júpiter would uiaintain the Min prc.-ont expcnditnre of cnergy K.r 32,-J54 ycars, and Mcrcury f'or six years aud 219 day, and that the èollapse of all the planets apOD the siiu would genérate sutlieient heat to uiaintan iu lupplj f newly 46,000 yen; and allüvating that n.atter eaual to only .,,„m om. „nr-l.mvlredtli part ui tlio luass of the earth, talling annually uon the solar Burfaoe, would uiaintainitsradiationmdefimtély. Prof. Voung. thinks it improbable IVoin istitnionikfel Peasonn, that any -u, h (iiiantity of matter con be Baprtoséa 10 reach the sun. So IT a ,uantny ol matter would neceüsitate a va.-tly greáter qTlantity circulating Monnd the un betweeu it and the planet Meirury. Hut it there wcre near the mui meleoric matter efiyallDg, lor Dxaiúpte, the iHHoftl.ecarth, it ought to prodttM an observable effect on the molióos ot' Mercury, aWd nb Won enen DM vet been detected. . -trononiers, therclore, lailing to luid a luíl explanation of the cause ut' solar cnergy in this hypothesis, ll;ive ud"lt('d a seconj one whieh is, üi;it the lilll diameter i.slowly contracting, and tliat the gaseoua mass is gradúa ly lirtuefying and uecomini volid. The conclusión is drawn that, if tliis theory be ooi reit, there muM cóme a Unie when there will be no solar heat. as theic bas al been li"1 hen 't began. llow far forward is the end, how tar baekward thebeginning? Newoomb is authority for the statemenl that, with iu preaeol rad,,,n. the mui will shrink to hall Lis present diameter in about i.000,000 jean. Iteduc „, thisswe, nd eight times as den aa ,ae to be man.ly gaseous, :.nd its teiuperature would begin to lall. Henee Xeweouib awignsasthe term dunnu whioh the Minean supply net enougn to support life on earib, as we knp;w lile, aperiod of 10,000,000 yeart. The wiitn Boniewhat more conndeutly casta liiseyc baekward, aud eludo tbal thesuncaiinotiiavc been eni'-ttkig heat at the present ratefot more than IS,(HK),(KIO yeara if its beat has boen generaled n Mie mannerdescribed. Ii-Uieunlasoontraci ed froni a diameter even many Uuics largcr than thal ol orbit to ris prCBenl dimensión-, as is probaUy tme in the. mam, '¦ ive. are mexorahly shut uu to tTw conclusión that 'he total lile of the solar systenj, trom its birtli WIta detb, is me.luded in somesuch 8Kiof luneas 30,000,000 O vears : mi reasonable allowanee lor Ibc lall of raeoH iimttêrj" e., "owU it ,,„,. The i.ossibllliy ot BOilw i ,n with #ahderin(? Mtu-, and the ¦ tioo ot' ways as yet unconeeitert ot loi restoring w:ifced energy, are follow.-.l by the slatumcut that " thC Jrcseil( onlcrol thinwappearstobc limit cd iu dthqr diretí tion by terminal oatastropbCH whieh fe veiled n ototida as yet impenetrable.