This the astronomer did. “Well,” he said, “what will this tell you? It merely expresses the amount of delinquency on the part of Encke.”
“No doubt,” said the mathematician, “that is so; but we must remember that the delinquency, as you call it, was caused by Mercury. The bigger and the heavier Mercury was, the greater would be his power of doing mischief, the more would he have troubled poor Encke, and the larger would be the derangement of the comet in consequence of the unfortunate incident. We have measured how much Encke has actually been led astray. Had Mercury been heavier than he is, that distance would have been larger; and if Mercury had been lighter than he is, you would not, of course, have found so large an error in the comet.”
We may illustrate what is meant in this way. A steamer sails from Liverpool to New York, and in favorable circumstances the voyage across the Atlantic should be accomplished within a week. But supposing that in the middle of the ocean a storm is encountered, by which the ship is driven from her course. She will, of course, be delayed, and her voyage will be lengthened. A trifling storm, perhaps, she will not mind, but a heavy storm might delay her six hours; a still greater storm might keep her back half a day; while cases are not infrequent in which the delay has amounted to one day, or two days, or even more.
The delay which the ship has experienced may be taken as a measure of the vehemence of the storm. I am not supposing that her machinery has broken down; of course, that sometimes happens at sea, as do calamities of a far more tragic nature. I am merely supposing the ship to be exposed to very heavy weather, from which she emerges just as sound as she was when the storm began. In such cases as this we may reasonably measure the intensity of the storm by the number of hours’ delay to which the passengers were subjected. “The weather we had was much worse than the weather you had,” one traveller may say to another. “Our ship was two days late, while you escaped with a loss of one day.”
When the comet at last returned to the earth after a cruise of three years through space, the number of hours by which it was late expressed the vehemence of the storm it experienced. The only storm that the comet would have met with, at least in so far as our present object is concerned, was the trouble that it had with Mercury. The mass of Mercury was, therefore, involved in the delay of the comet. In fact, the delay was a measure of the mass of the planet. I do not attempt to describe to you all the long work through which the mathematician had to plod before he could ascertain the mass of Mercury. It was a very tedious and a very hard sum, but at last his calculations arrived at the answer, and showed that Mercury must be a light globe compared to the earth. In fact, it would take twenty-five globes, each equal to Mercury, to weigh as much as the earth.
I dare say you will think that this was a very long and roundabout way of weighing. Supposing, however, we had to weigh a mountain, or rather a body which was bigger than fifty thousand mountains, and which was also many millions of miles away, all sorts of expedients would have to be resorted to. I have told you one of them. If you feel any doubts as to the accuracy with which such weighings can be made, then I must tell you that there are many other methods, and that these all agree in giving concordant results.
Fig. 49.—Relative Weights of Mercury and the Earth.
We hardly know anything as to what the globe of Mercury may be like. We can see little or nothing of the nature of its surface. We only perceive the planet to be a ball, brightly lighted by the sun, and we cannot satisfactorily discern permanent features thereon, as we are able to do on some of the other planets.