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Galilei, Galileo Dialogues on two world systems 1661, tr. Salusbury, Thomas

SALVIATUS, SAGREDUS, and SIMPLICIUS

( Salviatus: Galileo
  Sagredus: an intelligent layman
  Simplicius: an unthinking believer in Arisotle's system )


p. 164  (172 on web site)

SALV. Your discourse hath in it much of probability, and to 
stick at trivial doubts is not for an acute wit; yet neverthelesse the 
probability being removed, I believed that it hath not a jot more 
force than the others already considered and resolved.

SAGR. It is most certain that if it be not necessatily conclu-
dent, its efficacy must needs be just nothing at all, for it is 
onely when the conclusion is necessary that the opponent hath no-
thing to alledg on the contrary.

SALV. Your making a greater scruple of this than of the other 
instances dependeth, if I mistake not, upon the birds being ani- 
mated, and thereby enabled to use their strength at pleasure a-
gainst the primary motion in-bred in terrene bodies: like as for

p. 165  (173)

example, we see them whil'st they are alive to fly upwards, a thing 
altogether impossible for them to do as they are grave bodies; 
whereas being dead they can onely fall downwards; and there-
fore you hold that the reasons that are of force in all the kinds of 
projects above named, cannot take place in birds: Now this is 
very true; and because it is so, Sagredus, that doth not appear 
to be done in those projects, which we see the birds to do. For if
from the top of a Tower you let fall a dead bird and a live one, 
the dead bird shall do the same that a stone doth, that is, it shall 
first follow the general motion diurnal, and then the motion of 
descent, as grave; but if the bird let fall, be a live, what shall 
hinder it, (there ever remaining in it the diurnal motion) from 
soaring by help of its wings to what place of the Horizon it shall 
please? and this new motion, as being peculiar to the bird, and 
not participated by us, must of necessity be visible to us; and if 
it be moved by help of its wings towards the West, what shall 
hinder it from returning with a like help of its wings unto the 
Tower. And, because, in the last place, the birds wending its 
flight towards the West was no other than a withdrawing from 
the diurnal motion, (which hath, supppose ten degrees of velocity) 
one degree onely, there did thereupon remain to the bird whil'st 
it was in its flight nine degrees of velocity, and so soon as it did 
alight upon the the Earth, the ten common degrees returned to it, 
to which, by flying towards the East it might adde one, and with 
those eleven overtake the Tower. And in short, if we well con-
sider, and more narrowly examine the effects of the flight of 
birds, they differ from the projects shot or thrown to any part of 
the World in nothing, save onely that the projects are moved by an 
external projicient, and the birds by an internal principle. 

And 
here for a final proof of the nullity of all the experiments before 
alledged, I conceive it now a time and place convenient to 
demonstrate a way how to make an exact trial of them all. 
Shut your self up with some friend in the grand Cabbin between 
the decks of some large Ship, and there procure gnats, flies, and 
such other small winged creatures: get also a great tub (or 
other vessel) full of water, and within it put certain fishes; let 
also a certain bottle be hung up, which drop by drop letteth forth 
its water into another bottle placed underneath, having a narrow 
neck: and, the Ship lying still, observe diligently how those small 
winged animals fly with like velocity towards all parts of the Ca-
bin; how the fishes swim indifferently towards all sides; and how 
the distilling drops all fall into the bottle placed underneath. And 
casting any thing towards your friend, you need not throw it with 
more force one way then another, provided the distances be equal: 
and leaping, as the saying is, with your feet closed, you will reach

p. 166 (174)

as far one way as another. Having observed all these particulars, 
though no man doubteth that so long as the vessel stands still, they 
ought to succeed in this manner; make the Ship to move with 
what velocity you please; for (so long as the motion is uniforme, 
and not fluctuating this way and that way) you shall not discern 
any the least alteration in all the forenamed effects; nor can you 
gather by any of them whether the Ship doth move or stand still. 
In leaping you shall reach as far upon the floor, as before; nor for 
that the Ship moveth shall you make a greater leap towards the 
poop than towards the prow; howbeit in the time that you staid 
in the Air, the floor under your feet shall have run the contrary way 
to that of your jump; and throwing any thing to your companion 
you shall not need to cast it with more strength that it may reach 
him, if he shall be towards the prow, and you towards the poop, 
then if you stood in a contrary situation; the drops shall all distill 
as before into the inferiour bottle and not so much as one shall 
fall towards the poop, albeit whil'st the drop is in the Air, the Ship 
shall have run many feet; the Fishes in their water shall not swim 
with more trouble towards the fore-part, than towards the hinder 
part of the tub; but shall with equal velocity make to the bait 
placed on any side of the tub; and lastly, the flies and gnats 
shall continue their flight indifferently towards all parts; nor 
shall they ever happen to be driven together towards the side of 
the Cabbin next the prow, as if they were wearied with fol-
lowing the swift course of the Ship, from which through their 
suspension in the Air, they had been long separated; and if 
burning a few graines of incense you make a little smoke, 
you shall see it ascend on high, and there in manner of a cloud 
suspend it self, and move indifferently, not inclining more to one 
side than another: and of this correspondence of effects the cause 
is for that the Ships motion is common to all the things contained 
in it, and to the Air also; I mean if those things be shut up in the 
Cabbin: but in case those things were above deck in the open Air, 
and not obliged to follow the course of the Ship, differences more 
or lesse notable would be observed in some of the fore-named ef-
fects, and there is no doubt but that the smoke would stay behind 
as much as the Air it self; the flies also, and the gnats being hin-
dered by the Air would not be able to follow the motion of the 
Ship, if they were separated at any distance from it. But keeping 
neer thereto, because the Ship it self as being an unfractuous Fa-
brick, carrieth along with it part of its neerest Air, they would 
follow the said Ship without any pains or difficulty. And for the 
like reason we see sometimes in riding post, that the troublesome 
flies and ^{*} hornets do follow the horses flying sometimes to one, 
sometimes to another part of the body, but in the falling drops

p. 167 (175)

the difference would be very small; and in the salts, and projecti-
ons of grave bodies altogether imperceptible.

The answer to 
the argument ta-
ken from the flight 
of birds contrary 
to the motion of the 
Earth.

An experiment 
with which alone 
is shewn the nullity 
of all the objecti-
ons produced a­ 
gainst the motion 
of the Earth.

* Tafaris, borse-
flyes.

SAGR. Though it came not into my thoughts to make triall of 
these observations, when I was at Sea, yet am I confident that they 
will succeed in the same manner, as you have related; in confirma-
tion of which I remember that being in my Cabbin I have asked 
an hundred times whether the Ship moved or stood still; and 
sometimes I have imagined that it moved one way, when it steered 
quite another way. I am therefore as hitherto satisfied and con-
vinced of the nullity of all those experiments that have been pro-
duced in proof of the negative part. There now remains the ob-
jection founded upon that which experience shews us, namely, that 
a swift Vertigo or whirling about hath a faculty to extrude and 
disperse the matters adherent to the machine that turns round; 
whereupon many were of opinion, and Ptolomy amongst the rest, 
that if the Earth should turn round with so great velocity, the 
stones and creatures upon it should be tost into the Skie, and 
that there could not be a morter strong enough to fasten buildings 
so to their foundations, but that they would likewise suffer a like 
extrusion.