from his eyes, because the illumination is sufficiently strong: but when there is no vi sible illumination at all, how should it account for the like effect? whence it is more reasonable that this appearance should be owing to a light from within the body of the animal, which being weaker than the light of the day, but stronger than the light of the night, is visible in the night and not in the day. The light of those other bodies which shine in the dark, is inherent in the bodies themselves; as in putrifying veal, fish, rotten wood, phosphorus, the glow-worm, &c. concerning the last of which, that eminent anatomist and philosopher, T. Bartholine, hath this observation-Habent illæ (cicindelæ) si examinentur, lucidum humorem in posteriore parte corpusculi, ubi cor latet, quo movetur cor & illuminatur, tamdiu splendore suo relucentem, quamdiu cor vivit & movetur. Si tam patulum oculis nostris esset pectus animalium, quam cicindela transparens corpuscu✅lum, forsan non absimile lumen in corde appareret, quod extinguitur quamprimum externo aeri patet, &c. If a glow-worm be examined, it will appear to have a lucid liquor “in the hinder part of its body, where the heart "is placed, by which the heart is moved and "illumi

66

* illuminated; and this fluid retains its light, "so long as the heart of the insect retains its

life and motion. Were the breasts of ani"mals as pervious to the sight as the transpa"rent body of the glow-worm, perhaps a light, "analogous to this, might appear about the "region of the heart, which light is extinguished the moment it is exposed to the open T. Barthol. Epist. de Flammulâ

" air."

Cordis. p. 4.

ON CHAP. III.

An Addition to the Note at page 232-233.

SPEAKING of the force that is exerted in the experiments on artificial freezing, I have called it a force which art would find it difficult to measure. Such I then imagined it to be, and contented myself with relating an experiment, from which it could only be guessed at in the grossest manner. But being desirous of obtaining some certainty in an affair which deserves a deeper examination, I contrived

VOL. VIII.,

D D

trived a way, this last winter, of measuring this force to a sufficient degree of exact

ness.

I made use of the bor already described in the foregoing part of this note. It was filled, as before, with water purged of its air ; and being covered, but not screwed down, it was placed upon an oaken pedestal, which had for its base a flat. hewn stone of about a foot square. The shorter arm of a very strong lever was made to press upon the top of it; and this lever was compounded with two others, by which the power of it was greatly increased. At the extremity of the longer arm of the most remote lever, a cord was fastened, which ran over a pulley, and had a weight of 281b. hanging at the end of it. This weight, acting as a mechanical power, made the shorter arm of the first-mentioned lever press down the cover of the box with a certain force, which, according to the general law of all mechanics, was to the 28lb. at the end of the line, as the space moved through by the power, to the space moved through in the same time by the weight: but these spaces being compared with each other, I found that the greater was to the lesser as 82 to 1. So that if we multiply 28 by 82 the

product

product will be 2296 lb. to which something must be added for the friction of the machine, though I took what care I could to render its motion as true and easy as possible. While the cover of the box was pressed down by so great a weight, the water inclosed within it was made to freeze; and the agent by which the water was congealed, did fairly overcome the whole force of the machine.

Though this experiment succeeded so far as to give me the satisfaction I wanted, it was interrupted by an accident I did not look for. When the water began to freeze, and the cover of the box to be raised up from the rim, I perceived that the ground yielded under the pressure, and that the flat stone which served as a basis to the pedestal was sunk a little below its first position. By this means the force was at first spent upon the ground, and did not take place in the machine till the ground would no longer give way. It was, however, so sensibly perceived in the machine likewise, as to prove that this force was at least superior to one ton two hundred and ninety-six pounds. How much greater it may prove to be, I cannot as yet affirm; though I believe my apparatus would answer

the purpose if it were required to pursue the experiment to the bottom. But there are so many circumstances to be nicely attended to in order to make it succeed, and so much time and trouble to be bestowed upon it out of doors in cold weather, that my patience was wearied out, for this winter at least, by a single attempt.

The cavity of the box, as I find by gauging it, contains 5 cubic inches and . Water, when it freezes, commonly increases in its bulk by of the whole: therefore the space occupied by the æther, which exerted all this force upon the machine, was equal only to about of a cubic inch. And hence it will appear, by a calculation not worth particularizing, that the force of this æther (even in this experiment, which undoubtedly falls very far short of the whole truth,) was about two hundred and seventy-three times greater than the ordinary pressure of the atmosphere.

Mr. B. Robins, the engineer, found, by several accurate experiments, that the force of fired gunpowder is about 1000 times greater than the pressure of the atmosphere and the force of that agent which consolidates water into ice, may be found equal to it,

for