454 Rev. Maxicell H. Close — Extent of Geological Time. 



earth's figure." "But though the testimony of these two tides is 

 still so doubtful, let us accept it." The body of the earth yields 

 very little to a fortnightly or a monthly change of straining force. 

 But what about the 18-6 year variation of force connected with the 

 revolution of the moon's nodes ? Sir William Thomson says, " The 

 absence from all the results of any indication of an 18-6 year tide 

 is not easily explained without assuming or admitting a considerable 

 degree of yielding." Observe how much clearer and stronger his 

 language is now than before. There being no perceptible 18-6 year 

 tide in the ocean, we conclude that the body of the earth must yield 

 nearly as much as water to the variation of tidal force having that 

 long period, though it will only yield a very little to a fortnightly 

 or a monthly variation. The meaning of that is, that the rigidity of 

 the earth, though so high as to amount, is as to quality a viscous 

 rigidity. The peculiar character of a viscous solid is well known 

 and illustrated by a stick of sealing wax. The earth, then, may 

 yield as much as geologists need desire to the continued decrease of 

 the centrifugal force of rotation. And therefore it may be hundreds 

 of millions of years since she became solid, although her shape now 

 so nearly corresponds to her present rate of rotation. There are 

 quite independent considerations which make it most probable that 

 the rigidity of substances at a much higher temperature than that of 

 free fusion, but kept solid by pressure, is generally a viscous rigidity. 



I would beg leave to suggest a method by which the yielding of 

 the earth to the 18-6 year change of tidal force could in all probability 

 be verified. If there be, as there certainly seems to be, " a consider- 

 able degree of yielding " to this in the body of the earth, it consists 

 of an alternate rising and sinking of the whole equatorial belt of our 

 globe together. This must alternately diminish and increase the 

 rate of the earth's rotation — diminish it while the moon's ascending 

 node is passing (retrogressively) for 8-55 years, from about 15 deg. 

 W. of the winter to the same distance E. of the summer solstice, and 

 for lO'Oo years v.v. I believe that this change, though very small, 

 is of an order of magnitude that could be detected by the astronomers 

 if they would kindly undertake to look for it. They could not use 

 the moon as their timepiece, her tables not being yet as correct as 

 would be necessary in this case. But they have promised us revised 

 tables of the Satellites of Jupiter, and when they shall have fulfilled 

 their promise the innermost satellite would doubtless make a time- 

 piece sufficiently accurate for the purpose. 



Note. — The following coincidence seems worth mentioning, though 

 doubtless only accidental. Professor Newcomb has thrown out the 

 suggestion that certain unexplained irregularities in the moon's 

 motion may be only apparent and caused by inequalities in the 

 earth's rate of rotation. He has concluded that, if this be so, the 

 earth was going slow in her rotation from 1850 to 1862, when she 

 began to go fast. Now, from the above-mentioned cause the earth 

 would go slow from November 26, 1852 (or very shortly after), to 

 March 12, 1862 (or very shortly after), when, she would begin to go 

 fast again. The near coincidence is curious. But that is all ; for, 



