Foundations of the Universe 295 



ing down, taking longer to rotate on its axis, but the month in- 

 creased at a greater rate than the day. Presently the month 

 became equal to two days, then to three, and so on. It has been 

 calculated that this process went on until there were twenty-nine 

 days in the month. After that the number of days in the month 

 began to decrease until it reached its present value or magnitude, 

 and will continue to decrease until once more the month and t he- 

 day are equal. In that age the earth will be rotating very slowly. 

 The braking action of the tides will cause the earth always to keep 

 the same face to the moon; it will rotate on its axis in the same 

 time that the moon turns round the earth. If nothing but the 

 earth and moon were involved this state of affairs would be final. 

 But there is also the effect of the solar tides to be considered. The 

 moon makes the day equal to the month, but the sun has a ten- 

 dency, by still further slowing down the earth's rotation on its 

 axis, to make the day equal to the year. It would do this, of 

 course, by making the earth take as long to turn on its axis as to 

 go round the sun. It cannot succeed in this, owing to the action 

 of the moon, but it can succeed in making the day rather longer 

 than the month. 



Surprising as it may seem, we already have an illustration of 

 this possibility in the satellites of Mars. The Martian day is 

 about one half -hour longer than ours, but when the two minute 

 satellites of Mars were discovered it was noticed that the inner 

 one of the two revolved round Mars in about seven hours forty 

 minutes. In one Martian day, therefore, one of the moons of 

 Mars makes more than three complete revolutions round that 

 planet, so that, to an inhabitant of Mars, there would be more 

 than three months in a day. 



BIBLIOGRAPHY 



ARRHENIUS, SVANTE, Worlds in the Making. 



CLERK-MAXWELL, JAMES, Matter and Motion. 



DANIELL, ALFRED, A Text-Book of the Principles of Physic*. 



