Note i — BOOK I.

Fishes by reason of the Bladder of Air within them, can sustain, or keep themselves in any Depth of Water: For the Air in that Bladder being more or less compressed, according to the Depth the Fish swims at, takes up more or less Space; and consequently, the Body of the Fish, part of whose Bulk this Bladder is, is greater or less according to the several Depths, and yet retains the same Weight. Now the Rule de Insidentibus humido is, that a Body, that is heavier than so much Water, as is equal in Quantity to the Bulk of it, will sink, a Body that is lighter will swim; a Body of equal Weight will rest in any part of the Water. By this Rule, if the Fish, in the middle Region of the Water, be of equal Weight to the Water, that is commensurate to the Bulk of it, the Fish will rest there, without any Tendency upwards or downwards: And if the Fish be deeper in the Water, the Bulk of the Fish becoming less by the Compression of the Bladder, and yet retaining the same Weight, it will sink, and rest at the Bottom. And on the other side, if the Fish be higher than the middle Region, the Air dilating it self, and the Bulk of the Fish consequently increasing, but not the Weight, the Fish will rise upwards and rest at the top of the Water. Perhaps the Fish by some Action can emit Air out of its Bladder——, and, when not enough, take in Air,——and then it will not be wondred, that there should be always a fit Proportion of Air in all Fishes to serve their Use, &c. Then follows a Method of Mr. Boyl to experiment the Truth of this. After which, in Mr. Lowthorp’s Abridgment, follow Mr. Ray’s Observations. I think that——hath hit upon the true Use of the Swimming-Bladders in Fishes. For, 1. It hath been observed, that if the Swimming-Bladder of any Fish be pricked or broken, such a Fish sinks presently to the Bottom, and can neither support or raise it self up in the Water. 2. Flat Fishes, as Soles, Plaise, &c. which lie always grovelling at the Bottom, have no Swimming-Bladders that ever I could find. 3. In most Fishes there is a manifest Chanel leading from the Gullet——to the said Bladder, which without doubt serves for the conveying Air thereunto.——In the Coat of this Bladder is a musculous Power to contract it when the Fish lifts. See more very curious Observations relating to this Matter, of the late great Mr. Ray, as also of the curious anonymous Gentleman in the ingenious Mr. Lowthorp’s Abridgment, before cited, p. 845. from Phil. Trans. N. 114, 115.