Page:Mathematical collections and translations, in two tomes - Salusbury (1661).djvu/251

 I may then, it seems, for once prove a Master to you both. And because the proceeding by interrogatories doth in my opinion much dilucidate things, besides the pleasure which it affords of confounding our companion, forcing from him that which he thought he knew not, I will make use of that artifice. And first, I suppose that the Ship, Gally, or other Vessel, which we would discover, is a great way off, that is, four, six, ten, or twenty * miles, for that to kenne those neer at hand there is no need of these Glasses: & consequently, the Telescope may at such a distance of four or six miles conveniently discover the whole Vessel, & a much greater bulk. Now I demand what for species, & how many for number are the motions that are made upon the round top, depending on the fluctuation of the Ship.

We will suppose that the Ship goeth towards the East. First, in a calme Sea, it would have no other motion than this of progression, but adding the undulation of the Waves, there shall result thence one, which alternately hoysting and lowering the poop and prow, maketh the round top, to lean forwards and backwards; other waves driving the vessel sidewayes, bow the Mast to the Starboard and Larboard; others, may bring the ship somewhat abovt, and bear her away by the Misne from East, one while towards the * Northeast, another while toward the Southeast; others bearing her up by the Carine may make her onely to rise, and fall; and in sum, these motions are for species two, one that changeth the direction of the Telescope angularly, the other lineally, without changing angle, that is, alwayes keeping the tube of the Instrument parallel to its self.

Tell me, in the next place, if we, having first directed the Telescope yonder away towards the Tower of * Burano, six miles from hence, do turn it angularly to the right hand, or to the left, or else upwards or downwards, but a † straws breadth, what effect shall it have upon us touching the finding out of the said tower?

It would make us immediately lose sight of it, for such a declination, though small here, may import there hundreds and thousands of yards.

But if without changing the angle, keeping the tube alwayes parallel to it self, we should transfer it ten or twelve yards farther off to the right or left hand, upwards or downwards, what alteration would it make as to the Tower?

The change would be absolutely undiscernable; for that the spaces here and there being contained between parallel rayes, the mutations made here and there, ought to be equal, and because the space which the Instrument discovers yonder, is capable of many of those Towers; therefore we shall not lose sight of it.

Returning now to the Ship, we may undoubtedly affirm, that the Telescope moving to the right or left, upwards, or