A continuation of new experiments physico-mechanical, touching the spring and weight of the air and their effects. The I. part whereto is annext a short discourse of the atmospheres of consistent bodies / written by way of letter to the right honourable the Lord Clifford and Dungarvan by the honourable Robert Boyle ...

EXPERIMENT XXXI.

About the Attractive virtue of the Loadstone in an Exhausted Receiver.

SOme Learned modern Philosophers, that have attempted to explicate the cause and manner of Magnetical Attraction or Coition, give such an account of it, as supposes, that the Air be∣tween the two Magnetical Bodies, being driven away by their Ef∣fluviums from between them, presses them on the parts opposite to those where the Contact is to be made; and upon some such score (for I must not now stay to deliver their Theories Circum∣stantially) the Air is suppos'd to contribute very much to the At∣traction and Sustentation of the Iron by the Loadstone: where∣fore partly to examine this Opinion, and partly for some other Purposes (not necessary now to be mentioned) we thought fit to make the following Exptriment.

We took a small but vigorous Loadstone, cap'd and fitted with a loose plate of Steel, so shap'd, that when it was sustained by the Loadstone, we could hang at a litle Crook, that came out of the midst of it, and pointed downwards, a Scale, wherein to put what Weights we should think fit. Into this Scale we put sometimes more and sometimes less weight, and then by shaking of the Load∣stone as much as we guess'd it would be shaken by the motion of the Engine, we found the greatest weight, that we presum'd it would be able to support, in spite of the Agitation 'twould be exposed to, which prov'd to be, besides the Iron-plate and the Scale, VI Ounces Troy weight, to which if we added half an ounce more, the whole weight appear'd too easie to be shaken off. This done, we hung the Loadstone, with all the weight it sustain'd, at a Button of Glass, which we had procur'd to be fastned on to the top of the inside of a Receiver, when 'twas first blown, and though in about 12 Exuctions we usually emptied such Receivers as

as much as was requisite for most Experiments; yet this time, to exhaust it the more accurately, we continued pumping till we had exceeded twice that number of Exuctions, at the end of which time shaking the Engine somewhat rudely, without thereby sha∣king off the Weight that hung at the Loastone, the Iron seem'd to be very near as firmly sustain'd by it as before the Air began to be pump'd out. I said very near, rather than altogether, be∣cause that the withdrawing of the Air, though it be not suppos'd to weaken at all the Power of the Loadstone precisely considered, yet it must lessen its power to sustain the Steel, because this in so thin a medium must weigh heavier, than in the Air, by the weight of as much Air, as is equal in bulk to the appended Body.

Some other Magnetical Tryals (and also some Electrical ones) I remember I attempted to make by the help of our Engine, but not having the Notes I took of them now at hand, I shall su∣spend the mentioning them, till I can give Your Lordship a more punctual Account of them.