VOL. IX.] PHILOSOPHICAL TRANSACTIONS. 205 



of equality here is sure by a kind of demonstration altogether geometrical, and 

 withal vei-y evident even to ordinary capacities. 



It remains to notice, in a few words, the objections that have been made 

 against this contrivance by some intelligent persons. They have all acknow- 

 ledged, that this would be a perfectly exact watch for common use; but if em- 

 ployed for finding the longitudes, there would occur these difficulties, viz. that 

 tossing of ships would shake the springs as well as other pieces; that rust would 

 spoil them, since the saltish humidity of the sea in remote voyages spares not the 

 very needles of compasses, though enclosed in boxes ; that the changes of 

 seasons and climates will sensibly alter the springs, especially the great heats, or 

 rains within the tropics, which at length will somewhat untemper the steel ; as 

 is confirmed by the experiments of the illustrious academy of Florence, showing 

 how easily heat and cold change slender springs : besides that the air, more or 

 less condensed, will also more or less resist the motion of the balance. To 

 which may be added, that springs are weakened by working; and lastly, that 

 there will be always some little friction, that will make the several pieces go 

 more or less easily, and that even in length of time they will wear out. 



But I answer, that all these defects, that proceed from the imperfection of the 

 matter, may be surmounted by a general remedy, without examining them here 

 in particular. And that is, that for executing it in great, we may make use of 

 massy springs, as are those of cross-bows, we being masters of them, not want-__, 

 ing force or place in a ship to govern a great weight that may serve to bend 

 them continually again. Now these massy springs may be so great, and their 

 restitution so speedy by augmenting their number, that all the above named 

 defects will have no colisiderable proportion to this strength, and the aggregate 

 of their repetitions will not be sensible till after a very long time. And it is easy 

 to demonstrate, that by augmenting the size of the engine, and the force of the 

 massy springs, we may make the error as small as we please, provided we pass 

 not the bounds of conveniency, and content ourselves with an exactness suffi- 

 cient for the end they are principally designed for, which is the finding of the 

 longitudes : which answer is so clear and so universal, that all those that have 

 considered it have expressed their satisfaction therein. 



Ohservatio EcUpseos Lunce totalis cum Occultationihus quanindam Fixarum, habita 

 d Joh, Hevelio, Anno 1675, die 9,11 Januarii S. N. Fesp. N° 113, p. 289. 



This eclipse was observed by Hevelius, atDantzic, as follows: 



Beginning of the eclipse . , 6h. 41m. 50s. 



Total immersion 7 42 44 



