Proctor's Astronomical Tjecfuret. 



II 



Interest. A simple method will enable us-- to remember 

 readily tin- various (acts regarding tho planets. I divide 

 the solar family into two parts tho Interior family, 

 Mercury. Venus, the earth and moon, a nil Mars ; and the 

 outer family, Jirnter, Saturn, Uranus, and Neptune. 

 Quo of tlie facts is tho utter diversity of char- 

 acter between these two families. I want to show 

 that instead of being regarded as two similar 

 families, they ought to be looked upon as two sep- 

 arate families, utterly unlike each other. Whatever 

 we think of the outer family of larger planets ought to bo 

 derived from the evidences wo have, and not from the 

 analogy of our own earth. It is only tho inner family of 

 planets that wo can judge by our own earth. Ana even 

 in this case we must not fall into the mistake that life 

 on other worlds must be like that of our earth. Our 

 earth itself eives us some such lesson. If we saw one 

 part, and heard of but were prevented from seeing the 

 oiher _>arts, ws might think that life would 

 not exist uudcrtbe varying conditions. Yet we know it 

 does. Even very recently tliero was a very striking 

 case in illustration. We all thought that there could 

 be no life at the bottom of the sea ; that in the deepest 

 peas there was a great darkness and a pressure which 

 must prevent life. At the depth of a mile or two the 

 pressure is so great that when wood is carried down the 

 water is forced into it, so that it will never float again. 

 We know also that light is a desirable quality for all 

 living creatures we are acquainted with, aod we 

 came to the conclusion that light could not exist 

 there. But Drs. Carpenter and Thompson have let down 

 their dredges and drawn up, from the depth of two or 

 three miles, living creatures and not merely living 

 creatures, but having eyes and able to see, not- 

 withstanding the darkness that was supposed to reign 

 there. So unlike are the conditions there, that when 

 these creatures were brought up where the pressure was 

 less, they burst. There was quite enough remaining to 

 show that they had been alive but had been torn apart 

 when the pressure diminished. 



That was one example. We might also consider 

 the Arctic regions, and say how unlikely it would l>e to 

 anybody in the temperate zones that life could exist 

 there. How utterly unlikely, again, that life should 

 exist in the torrid zone ; or, a train, when you climb the 

 higiits of mountains, and coma to places where the at- 

 mospheric pressure is very much diminished, and great 

 cold prevails, and all the conditions are unlike those 

 that exist at the sea level, you would be certain nc life 

 could exi-;t there, if it were not for the fact that we visit 

 those regions and ascertain that life does exist there. 

 So that in dealing with the different planets we need 

 iiot concern ourselves to show that the conditions are 

 absolutely lik* those prevailing on the earch. But it 

 will be a useful thing to compare the conditions of 

 things as they exist in those planets with what prevails 

 on the earth. 



In order to get rid of those numbers, which will be 

 found in thf text-books of astronomy, lot us take 

 re.ative conceptions as to the distance. If you call the 

 distance of the earth 10, tiien for the distance of Mer- 

 cuiy from tho sun you have the number!; Venus 7, 



and Mars 16. For the diameters of those planets we 

 have Mercury 3,000 miles ; Venus, 7,500 miles ; for tho 

 earth, 7,900, and for Mars, 4,500. You see we have an 

 increase upward to tho earth and then downward. 

 Mercury being 3,000, Venua 7,500, tho earth 7.90D tho 

 largest of all these planets, and also dlgnlflo:! by having 

 a moon ; and then we have Mars with its diameter of 

 4,500 miles. All these planets rotate on their axes In 

 about 24 hours. They all resemble each other in that 

 respect. They all seem to have very similar densities, 

 to be composed of matter of about the same density aa 

 of this earth, tho density of our earth being about 4J 

 times, some say as much as 6.J times the density of water. 

 1C is somewhere between these values. 



MARS 



ORBITS OF THE INNER PLANETS. 



To begin with Mercury, the nearest, of all those 

 planets to the sun. Tue feature waich strikes u* first ia 

 dealing with Mercury is tho great heat to which that 

 planet is exposed. Mercury travels on an eccentric 

 orbit, and is exposed to a greater heat from the sun 

 at certain times than at others. We have before us dia- 

 grams of the orints. This inner orbit is the orbit of 

 Mercury, and you will notice Mercury is at one time 

 much nearer to the sun tnan in another part of his year. 

 The year of Mercury is 88 of our days, so that in the 

 course of 88 days Mercury passes from a verv great hoat 

 when nearest, to a comparatively less heat when 

 farthest from the sun. But, even when the sun's heat, is 

 least, it is much greater than that to which our earth is 

 exposed. Tho quantity of heat received by Mer- 

 curv varies from four times to ten times what wo 

 have. Now, that is really a serious difference. Only 

 imagine what would happen to us if the sun's light and 

 beat were increased four-fold, and then extend your 

 conception to an increase ten-fold. I think I need 

 hardly say that the heat in that case would bo so great 

 that creatures such as we are could not exist ; animal 

 life would be destroyed on the earth if the sun suddenly 

 gave out from four to ten times as much heat as ho 

 actually does. Then, can there be inhabitants in Mer- 



