nd _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 _cohaere_ and _adhaere_.
Adhaerence connotes a one-sided force (an impossibility in physics),
cohaerence a mutual force.

[177] PAGE 90, LINE 9. Page 90, line 9. _nempe vt alter polus maius pondus
arripiat._--This acute observation is even now not as well known as it
ought to be. Only so recently as 1861 Siemens patented the device of
fastening a mass of iron to one end of an electromagnet in order to
increase the power of the other end. The fact, so far as it relates to
permanent magnets was known to Servington Savery. See _Philos.
Transactions_, 1729, p. 295.

[178] PAGE 92, LINE 3. Page 92, line 4. _Suspendit in aere ferrum Baptista
Porta._--Porta's experiment is thus described (_Natural Magick_, London,
1658, p. 204): "_Petrus Pellegrinus_ saith, he shewed in another work how
that might be done: but that work is not to be found. Why I think it
extream hard, I shall say afterwards. But I say it may be done, because I
have now done it, to hold it fast by an invisible band, to hang in the air;
onely so, that it be bound with a small thread beneath, that it may not
rise higher: and then striving to catch hold of the stone above, it will
hang in the air, and tremble and wag itself."

[179] PAGE 97, LINE 29. Page 97, line 33. _Sed quaeri potest ..._--The
question here raised by Gilbert is whether the lifting-power of magnets of
equal quality is proportional to their weight. If a stone weighing a drachm
will lift a drachm, would a stone that weighs an ounce lift an ounce?
Gilbert erroneously answers that this is so, and that the lifting-power of
a loadstone, whether armed or unarmed, is proportional to its mass.

The true law of the tractive force or lifting-power of magnets was first
given in 1729 by James Hamilton (afterwards Earl of Abercorn) in a work
entitled _Calculations and Tables Relating to the Attractive Virtue of
Loadstones ... Printed_ [at London?] _in the Year_ 1729. (See also a paper
in the _Philos. Transactions_, 1729-30, vol. xxxvi., p. 245). This work
begins thus:

"The Principle upon which these Tables are formed, is this: That if Two
_Loadstones_ are perfectly Homogeneous, that is, if their Matter be of the
same Specifick Gravity, and of the same Virtue in all Parts of one Stone,