[169] Page 85, line 26. Page 85, line 31. non conveniant.—The editions of 1628 and 1633 both read et conveniant.
[170] Page 86, line 3. Page 86, line 3. illud quod exhalat.—Literally, that which exhales, in the sense of that which escapes: but in modern English the verb exhale in the active voice is now not used of the substance that escapes, but is used of the thing which emits it. It must therefore be rendered that which is exhaled (i.e., breathed out).
[171] Page 86, line 13. Page 86, line 15. Ita tota interposita moles terrestris.—Gilbert's notion that the gravitational force of the moon in producing the tides acts through the substance of the earth may seem curiously expressed. But the underlying contention is essentially true to-day. The force of gravity is not cut off or screened off by the interposition of other masses. A recent investigation by Professor Poynting, F.R.S., has shown that so far as all evidence goes all bodies, even the densest, are transparent with respect to gravitational forces.
[172] Page 86, line 18. Page 86, line 20. Sed de æstus ratione aliàs.—There is no further discussion of the tides in De Magnete. But a short account is to be found in Gilbert's posthumous work De Mundo nostro Sublunari Philosophia nova (Amsterdam, Elzevir, 1651), in Lib. v., the part which in the manuscript was left in English, and was turned into Latin by his brother. It comprises about fifteen quarto pages, from Cap. X. to Cap. XIX. inclusive, beginning with a characteristic diatribe against Taisnier, Levinus Lemnius, and Scaliger. But in assigning causes he himself goes wide of the mark. Proceeding by a process of elimination he first shows that the moon's light cannot be the cause that impels the tides. "Luna," he says, "non radio, non lumine, maria impellit. quomodo igitur? Sane corporum conspiratione, acque (ut similitudine rem exponam) Magnetica attractione." This cryptic utterance he proceeds to explain by a diagram, and adds: "Quare Luna non tam attrahit mare, quàm humorem & spiritum subterraneum; nec plus resistit interposita terra, quàm mensa, aut quicquam aliud densum, aut crassum, magnetis viribus."
[173] Page 87, line 7. Page 87, line 9. armatura.—Here this means the cap or snout of iron with which the loadstone was armed. This is apparently the first use of the term in this sense.
In the Dialogues of Galileo (p. 369 of Salusbury's Mathematical Collections, Dialogue iii.), Sagredus and Salviatus discuss the arming of the loadstone, and the increased lifting power conferred by adding an iron cap. Salviatus mentions a loadstone in the Florentine Academy which, unarmed, weighed six ounces, lifting only two ounces, but which when armed took up 160 ounces. Whereupon Galileo makes Salviatus say: "I extreamly praise, admire, and envy this Authour, for that a conceit so stupendious should come into his minde. ... I think him [i.e., Gilbert] moreover worthy of extraordinary applause for the many new and true Observations that he made, to the disgrace of so many fabulous Authours, that write not only what they do not know, but whatever they hear spoken by the foolish vulgar, never seeking to assure themselves of the same by experience, perhaps, because they are unwilling to diminish the bulk of their Books."
[174] Page 87, line 12. Page 87, line 15. The reference to lib. 3 is
a misprint for lib. 2. It is corrected in the edition of 1633, but not in that of 1628.
[175] Page 87, line 17. Page 87, line 21. conactu.—The editions of 1628 and 1633 read conatu.
[176] Page 88, line 2. Page 88, line 3. Coitio verò non fortior.—This heading to chap. xix., taken with the seven lines that follow, and the contrast drawn between unitio and coitio, throw much light on the fundamental sense attached by Gilbert to the term coitio. It is here clearly used in the sense of mutual tendency toward union. Note also the contrasted use in chap. xx. of the verbs cohære and adhære. Adhærence connotes a one-sided force (an impossibility in physics), cohærence a mutual force.