This Explication, being but a Simile without a distinct application to Particulars, is beside so very short, that I can at best but give a conjecture at the meaning; which if I mistake, I shall deserve to be excused, and expect to be better inform'd.
By the Bloods doing the Office of a Pondus, I suppose he means, that the Blood contributes in the same manner to the motion of the Heart, as the Weights do to that of the Pendulum of a Clock. If so, the Blood, according to him, must be the Instrument of Constriction; and Dilatation must be the Natural State, or Spontaneous Motion, to which it wou'd, when under no violence, return; the contrary of which, I presume, will appear e're I have done.
But if he means, that the Blood in its reflux, by gravitating on the Auricles and Ventricles, dilates and expands 'em, acting therein as a Counterpoise to its contractions as a Muscle, I cou'd wish his Design had not bound him up to so narrow a compass, and that he had given us an explication at large of so abstruse and so important a Phænomenon: Because the Specifick Gravity of the Blood seems to me a cause by no means alone adequate to the effect, which it is here suppos'd to produce.
For, if the Blood acts only as a weight by meer gravitation, then that part of it only which descends from the Parts above the Heart can be employ'd in that Action. This at the largest computation can't amount to Five pound weight, and must, according to the computation of Borellus, force a Machine, that is able to overcome a resistance of 135,000l. I leave every Man to deduct what he shall upon examination find reasonably to be deducted, and yet shall rest secure, that it is not to be effected in the least with so small a Weight.
But neither does the Refluent Blood gravitate in any such proportion, as I have here assign'd. For to make a true estimate of its Gravitation, we must consider the Circumstances of the Liquor suppos'd to gravitate; in which it very much resembles Water inclos'd in a recurve Tube, of which, if the length of the two Legs be equal, it may be suspended in the Air full of Water, with the Extremities downwards, without losing a drop, although the Diameter of those Legs should be very unequal. The Case of the Arteries and Veins is pretty near a parallel to a Tube, so fill'd and inverted. For, if the Arteries and Veins be continued Tubes, (as by the Microscope they are made to appear) then supposing their contents to have no other determination of motion, than their own weight wou'd give them, the contain'd Fluids must be Counterpoises to each other. For the Veins and Arteries being join'd at the smaller Extremities, and the larger of both terminating in the same parallel Line, it is impossible, according to the Laws of Hydrostaticks, that the contents of either shou'd overbalance t'other. How far then must it fall short of forcing the natural Power and Resistance of so strong a Muscle as the Heart, by meer Gravitation?
The Blood indeed has a Progressive Motion through its Vessels, wherein it differs from Water, in a recurve Tube, in the Experiment above-stated. But, if the natural Gravitation of the Blood contributes nothing to the Dilatation of the Heart, this progressive Motion will not be found much more sufficient. For, as this Motion is deriv'd intirely from the Heart's Constriction (as all Accounts hitherto derive it) cou'd the Blood be suppos'd to re-act upon it by the Heart, with all the force first impress'd upon it by the Heart, it would be insufficient, unless we will suppose the Force communicated to be superiour to the Power Communicant, which is absurd.
But when the just and necessary Deductions for the Impediments, which the Blood meets with in its Progress through the Vessels, shall be made, the remaining Force will be found so exceeding weak, that to prop the Blood through the Veins may be a task alone too great for so small a Power, without charging it with the additional difficulty of forcing the Muscle of the Heart.
Alphonsus Borellus, after a great deal of solemn pains taken to shew his Care and Exactness, and to possess his Reader of the Truth of his Calculations, casts up the force of the Heart, and the Muscular Coat of the Arteries, to be together equal to a weight of 3,750l. and allots them a Resistance equal to 180,000l. to overcome which is 45 to 1. To make up for a disproportion, by his own confession, incredible to those who have not consider'd the Matter as he had done, he flings into the Scale the additional Force of Percussion, which he leaves indefinite, and thinks sufficient to force any quiescent finite Resistance whatsoever.
But as this Account and Hypothesis are part of a Posthumous Work (if a liberty of Conjecture may be allow'd in so uncertain a Matter,) I shou'd suspect, that these Papers were left unfinish'd by Borellus; or at least, that in many places the last Hand was never put to them. For neither in this Place, nor any other of this Work, does he account for any more than the Systole of the Heart, and the resistance which is made to the progressive motion of the Blood in the Arteries only. This alone he found to exceed the Power of the Heart so prodigiously, that he seems to shuffle it off his Hands with a general and precarious Solution, as a difficulty that he was desirous to be rid of. For, having ascrib'd this stupendous (as he himself calls it) effect to the Energy of Percussion, he takes no care to satisfie his Reader any farther about it, or to refer him, or give him the expectation of Satisfaction any where else; although he has an express Treatise on the Force of Percussion, which was written preparatory to this, and to which he frequently refers in other Places of this Work. But what confirms my suspicion, that this part was intended for a farther Revise by the Author, is, that he has left the Progress of the Blood through the Veins, and the Diastole of the Heart, absolutely untouch'd, tho' they are Difficulties of a much greater magnitude than this, which he has attempted to account so slightly for: For, in these he is excluded the benefit of Percussion, and has yet a greater resistance to overcome without it. Omissions of this kind are so unusual with this Author, where-ever he knows himself to go upon sure grounds, that it is to me an Argument, that he doubted the sufficience of his Percussion, and reserv'd these important Phænomena for farther Consideration, without plunging himself into such an Absurdity, as to ascribe to Percussion any such Energy as to be able (so broken as it returns to the Heart) by its re-action to force that Power, from whence only it was at first deriv'd.
Dr. Lower, and Mr. Cowper, deliver their Opinions of the Cause of the Dilatation of the Heart so very short, and without any Arguments to support them, that by exposing them naked, they seem rather to discourse of it transiently, as Men oblig'd by the Nature of their Subjects to say something of it, than solicitous to give any full or satisfactory Account; and therefore I shall proceed no farther upon them here.