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An Attempt to Prove the Motion of the Earth by Observations
(Continued)

And this mistake I found proceeded from divers inconveniencies their wayes of observations were lyable to. As first from the shrinking and stretching of the materials wherewith their Instruments were made, I conceive a much greater angle then that of a minute may be mistaken in taking an altitude of fifty Degrees.

For if the Instruments be made of Wood, 'tis manifest that moyst weather will make the frame stretch, and dry weather will make it shrink a much greater quantity then to vary a minute: and if it be Metal, unless it be provided for in the fabrick of the Instrument accordingly, the heat of Summer, when the Summer observations are to be made, will make the Quadrant swell, and the cold of Winter will make it shrink much more then to vary a minute: Both which inconveniencies ought to be removed. Next the bending and warping of an Instrument by its own weight, will make a very considerable alteration.

And thirdly, the common way of Division is also lyable to many inconveniencies: And 'tis hardly possible to ascertain all the subdivisions of Degrees into minutes for the whole Quadrant, though that be not altogether impossible.

But I will suppose that they did foresee, and in some manner prevent all these inconveniencies, especially Ticho and Riccioli, who seem to have been aware thereof.

But there was one inconvenience which was worse then all the rest, which they seem not to have been sufficiently sensible of, from whence proceeded all their own mistakes, and their imposing upon others, and that was from their opinion that the sight of the naked eye was able to distinguish the parts of the object as minutely as the limb of the Quadrant (of what largeness soever) was capable of Divisions; whereas 'tis hardly possible for any unarmed eye well to distinguish any Angle much smaller then that of a minute: and where two objects are not farther distant then a minute, if they are bright objects, they coalesce and appear one, though I confess, if they be dark objects, and a light be interposed, the distance between them shall be visible, though really much less then a Second; and yet notwithstanding, my first assertion stands good; for though a bright object, as a candle or light at a distance, or a Star, or the like, can be seen by the eye, though its body do really not subtend an Angle of one third, yet it proceeds from a radiation (that is, from reflection and refraction together) in the air and in the eye, whereby the body thereof is represented to the naked eye some hundred times bigger then it really is.

That this is so, any one that will but carefully examine will find it true. It was, I doubt not, their extraordinary desire and care to be exact, that caused them to make their Instruments so large, and to subdivide them to such an exactness, as to distinguish, if possible, to Seconds; And I question not but that they used their utmost indeavour in directing the sight to the object: but since the naked eye cannot distinguish an Angle much smaller then a minute, and very few to a whole minute, all their charge and trouble in making and managing large Instruments, and in calculating and deducing from them, was as to this use in vain.

Hence I judged that whatever mens eyes were in the younger age of the World, our eyes in this old age of it needed Spectacles; and therefore I resolved to assist my eyes with a very large and good Telescope, instead of the common sights, whereby I can with ease distinguish the parts of an object to Seconds: and I question not but that this way may be yet made capable of distinguishing much more curiously, possibly even to some few Thirds.

This invention removed that grand inconvenience which all former observations were spoiled with: but there remained yet further this difficulty, How to make an Instrument large enough for this purpose, that I might be assured did not shrink, nor warp, nor stretch so much as to vary a Second; for such is the nature of all Materials that can be made use of for Instruments of the bigness I designed this, that 'tis almost impossible to make a moveable Instrument that shall not be subject to a variation of divers Seconds: It was therefore my next inquiry where I might fix this Archimedean Engine that was to move the Earth.

For the doing of which, I knew 'twas in vain to consult with any Writer or Astronomer, having never then heard of any person that had ever before that time had any thoughts thereof: and when I first propounded it to the Royal Society, 'twas look'd upon as a new thought, and somewhat extravagant, and hardly practicable, until upon hearing my explication, and the various wayes how it might be reduced into practise, it was at length judged possible, and desirable to be tryed.

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