538 PHILOSOPHICAL TRANSACTIONS. [aNNO I67O. 



in motion. 5. The bodies which we move do cease to move because they are 

 impeded. 6. A body successively receiving many determinations remains only 

 affected with the last. 7 • A free body cannot be determined to move in a curve 

 line, nor with unequal celerity. 8. Every body that moves about a centre en- 

 deavours to recede from it. g. How a body may be moved circularly, lo. One 

 body moving against another body gives it its whole motion. 11. In the meet- 

 ing of two bodies there is made a percussion which is mutual, and equally re- 

 ceived in both. 12. A moving body meeting another body that is quiescent, 

 gives it all its motion, and remains itself motionless. 13. What is meant by 

 absolute and respective velocity. 14. The percussions are as the respective ve- 

 locities. 15. Two bodies meeting one another, turn back, making an exchange 

 of their velocity. l6. Two bodies moving toward the same places, continue af- 

 ter their encounter by exchanging their velocities. 17. A hard body striking 

 another body that cannot be shaken is reflected with its whole motion. 18. The 

 angle of reflection is equal to the angle of incidence. 19. It may be imagined 

 that the oblique motion is composed of two motions. 20. A general rule for 

 all percussions. 21. There is always an equal quantity of respective motion. 

 22. The midst or centre of gravity of two bodies is always uniformly moved in 

 a direct line. 23. All these rules are true, whether the bodies be equal or un- 

 equal. 24. A body moves in pleno as freely as in vacuo. 25. The percussions 

 of equal bodies are made in pleno as in vacuo ; but when the bodies are unequal, 

 the percussions are made in pleno otherwise than in vacuo. 26. The percus- 

 sions of unequal bodies cannot be reduced to one general rule. 27. Of refrac- 

 tion. 28. An appendix containing a review of this discourse. 



IV. Congietture Physico Astronomiche della Natura del Universo, da Pietro 

 M. Cavina; in Faenza, 1669, in4to. 



These conjectures are raised by the author upon some celestial observations 

 about the fixed stars at Faenza. The whole tract consists of three parts, consi- 

 derations, observations, and reflections, all of an erroneous and extravagant 

 nature. 



V. Dimostratione Fisico Matematica delle sette Propositioni, che promesse 

 Donato Rosetti. In Firenze, 1668, in 4to. 



The seven propositions which the author pretends to have here demonstrated 

 are these: 



1. What is the true physical cause of equilibriums? 2. The doctrine of Ar- 

 chimedes, importing, that a floating body sinks beneath the level of the water 

 so far, as that a mass of water, equal to the part immersed, does absolutely 

 weigh as much as the whole floating body, is false. 3. It is very probable that 

 there is no aether, and that consequently there is a vast vacuum. 4. There is a 



