But after devoting so much time to "force-correlation," and "force-differentiation," the advocates of "molecular-machinery" may feel themselves neglected if we dismiss their favorite hobby without further notice. The precise parentage of this term is disputed, but it has any number of _putative_ fathers. We have spoken of the size of the molecules themselves, and the numbers of them that might be huddled together on the point of a cambric needle without jostling. Let us now consider the size of a molecular machine. For each molecule runs its own machine, and is provident enough to see that they do not jostle. In fact, it is a very nice question in physics, whether the machines do not run the molecules, instead of the prevailing opposite opinion that the molecules run the machines. Unfortunately, the question is one that can never be determined. The requisite scientific data will forever be wanting. But Professor James C. Maxwell, now, or quite recently, filling the chair of experimental physics in the University of Cambridge, England, has furnished us with _approximate_ calculations. On the strength of his approximations we will proceed to consider the dimensions of these wonderful little machines. And first, it may be axiomatically laid down that these molecular machines, which either run the molecules or are run by them, can never exceed the size of their respective molecules. Conceding, then, that each one of these machines exactly fits into its own molecule, so as to present identically the same dimensions--as well as their largest possible dimensions--it would require two millions of them, placed in a row, to make one millimetre, or the one three hundred and ninety-four thousandths of an inch in length, or seven hundred and eighty-eight billions of them to make one inch! Who will ever be staggered at _Sirius_-distances, after this? And who will deny that an infinite world lies below the point of our microscopic vision, if not an Infinite kingdom and throne beyond our telescopic glance? But, following the same high authority in experimental physics, let us consider the aggregate weight of these molecular machines. We will not marshal their aggregate numbers in a row, for an array of forty billions of them would make too insignificant a figure for inspection; but simply give their actual weight as computed under the French or metric system. Take, then, a million million million million of these machines, throwing in molecules and all, and they will weigh, if there is no indiscreet kicking of the beam, just a fraction between four and five grammes, or--to differentiate the weights--a small fraction over one-tenth of an ounce!