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1831.] appearance of being such. But some gentlemen have imagined there may be a deception in the case, and that they do not really turn round, though indeed they seem to do so. The doubt of these gentlemen arises from the difficulty they find in conceiving how or in what manner a wheel or any other form, as part of a living animal, can possibly turn upon an axis supposed to be another part of the same living animal, since the wheel must be a part absolutely distinct and separate from the axis whereon it turns; and then say they, how can this living wheel be nourished, as there cannot be any vessels of communication between that and the part it goes round upon, and which it must be separate and distinct from? To this I can only answer, that place the object in whatever light or manner you please, when the wheels are fully protruded they never fail to show all the visible marks imaginable of a regular turning round; which I think no less difficult to account for, if they do not really do so. Nay, in some positions you may, with your eye, follow the same cogs or teeth whilst they seem to make a complete revolution; for the other parts of the insect being very transparent, they are easily distinguished through it. As for the machinery, I shall only say, that no true judgment can be formed of the structure and parts of minute insects by imaginary comparisons between them and larger animals, to which they bear not the least similitude. However, as a man can move his arms or his legs circularly as long and as often as he pleases by the articulation of a ball and socket, may there not possibly be some sort of articulation in this creature whereby its wheels or funnels are enabled to turn themselves quite round?

"It is certain all appearances are so much on this side of the question, that I never met with any who did not, on seeing it, call it a rotation; though, from a difficulty concerning how it can be effected, some have imagined they might be deceived. M. Leeuwenhoek also declared them to be wheels that turn round (vide Phil. Trans., No. 295). But I shall contend with nobody about this matter: it is very easy for me, I know, to be mistaken, and so far possible for others to be so too, that l am persuaded some have mistaken the animal itself which perhaps they never saw; whilst, instead thereof, they have been examining one or other of the several water-animalcules that are furnished with an apparatus commonly called wheels, though