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one of the vesicles, and apply a large magnifier to the place, and he will discover a hole, by which this vesicle or ovary has had a communication through the skin with the parent polype. For a further illustration of the manner in which these vesiculated polypes breed, let him consult the 38th plate of my Essay, where he will find several accurate figures of these vesicles, with the spawn of the polypes coming out of them; some of which spawn we evidently discovered to be young polypes with their arms formed; and as they fell from the vesicle, extending themselves in the watch-glass of sea-water.

In examining the drawings for his plates, it appears that fig. 2 is evidently a red conferva, which he calls a coralline. We have no corallines, but many confervas of this form and bright red colour on our coasts; and these shores are allowed to have similar marine productions with those of Holland. Fig. 5 he calls a branch of red coralline, which he says he kept several weeks in sea-water, and that often changed; during which time it sprouted and grew very much. This experiment is probably very true; because it is plainly a vegetable, as appears from his own exact drawing of it; and seems to be the fucus teres rubens minus in longum protensus of Ray's Synopsis, ed. 3, p. 51, n. 53. This is one of his principal arguments to prove the vegetation of corallines. Fig. 9 he calls a branch of red coralline; and at fig. 10 he has it magnified, where it appears to be a geniculated red conferva, drawn and painted with great exactness.

These arguments, and these figures of real vegetables, which the Doctor has given us for corallines, show how much he is willing to support the old opinion of the botanists: but he will soon alter his opinion, when he observes the remarkable difference of the texture of vegetable and coralline bodies, when viewed in sea-water through a good aquatic microscope. And to convince him more fully that corallines are an animal substance, let him burn them, and he will perceive the same pungent volatile alkaline smell, which he finds in burning horn, hair, or oysters; whereas burnt fucuses and confervas yield a smell not much unlike that of common land vegetables. Even the stony corallines, when their cretaceous covering has been dissolved in vinegar, the membranous part that remains of them, put into the fire, yields the same animal smell with other corallines.

XXXIV. Of an Extraordinary Operation performed in the Dock-yard at Portsmouth. By Mr. John Robertson, F. R. S. p. 288.

This account of the operation goes to show the effect of wedges in raising ships, or any great weights. In the present instance Mr. R., by an ingenious calculation, finds how much of the ship's weight was lifted by each screw and wedge in particular he discovers that each man employed in the operation, with his mallet and wedge, must have lifted at each stroke about 2 tons weight of

the ship; which, he observes, is considerably less than he had seen raised by way of experiment. Besides, it may be observed, that the pressure is at least doubled by the friction.

XXXV. On an Evening, or rather Nocturnal, Solar Iris. By Mr. George Edwards, Librarian of the Coll. of Phys. p. 293.

On Sunday evening, June 5, 1757, walking in the fields near Islington, about half a mile north of the upper reservoir.or basin of the New River, Mr. E. observed the sun to sink beneath the visible horizon to the north-west, it being very clear in that quarter, except some thin clouds a little above the horizon, which were painted of fine red and golden colours, as is usual when the sun sets in a calm clear evening. But about 20 minutes after sun-set, it being then darkish, he saw an iris in the dusky air, at a height greater than is seen at any time in the rainbow. It was in the contrary quarter of the heavens to the setting sun, and fell on the smoke, mists, and evening vapours arising from the city of London and its neighbourhood. The arch seemed to be a full half circle, though its lower parts fell some degrees short of the horizon. It was very distinctly seen for about 15 minutes; its colours the same as in the rainbow, but fainter. The lower ends of the bow arose gradually higher from the earth, as the sun declined beneath the horizon, till the whole arch disappeared. The centre of the arch was above the horizon at its first appearance. He could not believe that this arch proceeded from the sun-beams falling on rain: for there had been none that afternoon; nor was there any sort of signs of rain or rainy clouds to be seen the wind being northerly, and the air cool, but somewhat hazy in the quarter where the bow appeared. It was not near so bright as the rainbow appears to be in the day time; and he believes that it would not have been visible at all in the presence of the sun. He imagines it was formed on the gross par ticles of the evening vapours, mixed with those of the smoke arising from the town. This could not be a lunar arch, the moon being then many degrees below the horizon, and the arch in a place, where it could not be affected by the moon's rays.

XXXVI. The Effects of the Opuntia, or Prickly Pear, and of the Indigo Plant, in Colouring the Juices of Living Animals. Communicated by H. Baker, F. R. S. p. 296.

Mr. Baker received a letter from Doctor Alexander Garden, of Charlestown in South Carolina, part of which he laid before the R. S., containing this


The Doctor tried the effects of the prickly pear in colouring the urine. He gathered some of the fruit, and gave 4 of the pears to a child of 3 years of age,

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and 6 pears to one of 5. The next morning the urine of both appeared of a very · lively red colour, as if tent wine had been mixed with clear water. The urine of the eldest was deeper coloured, and of a darker look: the youngest (who always naturally made clear urine) was of a more lively and beautiful red. Next day he gave 6 pears to a negro wench, who gave suck, and strictly forbad her suckling her child for 6 or 8 hours; and then taking some of her milk in a tea-cup, and setting it by for some hours, the cream had a reddish lustre, though it was very faint. He was led to this last experiment by an observation, which he made on the milk of cows, which had fed in an indigo-field: the indigo had not only tinged their urine blue, but the cream of the milk was of a most beautiful blue colour, and had a radiated appearance from the centre (Is it not hence probable, that the dye is the oily part of the plant?). The milk underneath was clear and white as usual..

XXXVII. Of an Extraordinary Shower of Black Dust that fell in the Island of
Zetland, Oct. 20, 1755.* By Sir Andrew Mitchell, of Westshore, Bart. p. 297.

From Orkney Sir A. was informed, that about the time of the earthquake at Lisbon, Nov. 1, 1755, the tides were observed to be much higher than ordinary. He received from Zetland a letter, dated 28th May, 1756, from Mr. William Brown, master of the grammar-school at Scalloway in that country, a sensible and observing man; stating, on October 20, 1756, between the hours of 3 and 4 in the afternoon, the sky being very hazy, as it uses to be before a storm of thunder and lightning, there fell a black dust over all the country. It was very much like lamp-black, but smelt strongly of sulphur. People in the fields had their faces, hands, and linen, blackened by it. It was followed by rain. Some people assign the cause of it to some extraordinary eruption of Hecla. And the same was confirmed afterwards by several other persons in Zetland. At the time, the wind was from the s.w. which does not seem to favour the opinion that the dust proceeded from an eruption of mount Hecla, which lies about N.w. from Zetland; unless it may be supposed, that a north wind happening just before had carried this dust to the southward, and the south-west wind immediately following had brought it back to the northward.

The distance from mount Hecla to Zetland is between 500 and 600 miles.

XXXVIII. Of some Thermometers for Particular Uses. By the Right Hon. the Lord Charles Cavendish, V. P.R.S. P. 300.

The thermometer, pl. 7, fig. 1, is designed for showing the greatest degree of heat, which happens in any place during the absence of the observer. It con

* See vol. 10, p. 687, of these Abridgements.


sists of a cylinder of glass joined to a tube, and differs from common thermometers only in having the top of the stem drawn out into a capillary tube, which enters into a glass ball c, joined on to the stem at the place where it begins to be contracted. The cylinder, and part of the tube, are filled with mercury; the top of which shows the common degrees of heat as usual. The upper part of the tube above the mercury is filled with spirit of wine, and some of the same liquor is left in the ball c, so as to fill it almost up to the top of the capillary tube. Now when the thermometer rises, the spirit of wine will be driven out of the tube, and will fall into the ball c. When the thermometer sinks again, as the spirit cannot return back from the ball, the top of the tube will remain empty, and the length of the empty part will be proportional to the fall of the thermometer. Therefore, by means of a proper scale, the top of the spirit of wine will show how many degrees it has been higher than when observed; which being added to the present height, will give the greatest degree of heat it 'has been at.

To fit this thermometer for a new observation, it is necessary to fill the upper part of the tube with spirits; which may be done by inclining the instrument till the spirits in the ball c cover the end of the capillary tube. For if the cylinder is then heated, by applying the hand to it, or by the flame of a lamp held at some distance, till the spirits rise to the top of the tube and run over into the ball c, and is then suffered to cool in the same position, the tube will remain full of spirits, and the thermometer will be fitted for a new experiment.

The top of the capillary tube is made to stand pretty near to one side of the ball, and also to the top of it, that a less inclination of the instrument may be 'sufficient to make the spirit of wine in the ball cover the end of the tube. The ball c is joined on as high as possible, so as to hide no part of the tubę, except that where the bore is contracted. By this means the top of the spirit of wine begins to appear before the thermometer has sunk one degree. It is convenient to leave some mercury in the ball c, which may be made to cover the end of the capillary tube, by inclining the thermometer more than what is necessary to make the spirit of wine cover it. By this means some mercury may be got back into the tube, in case any of it should happen to be driven into the ball by the thermometer's being exposed to too great a heat.

The scale of degrees at top, which shows the descent of the thermometer from the highest point it has arrived at, ought not in strictness to be the same at all times of the year; for those degrees exceed the common degrees of heat pointed out by the top of the mercury, as much as the column of spirit of wine expands, and therefore are greatest when that column is so; that is, when the greatest heat to which the instrument has been exposed is least. A difference of 30 degrees of Fahrenheit's scale, in the greatest rise of the thermometer,

would require the scale to be altered one 60th part: and the error arising from making use of the same scale will be about one 6th of a degree, if the thermometer is observed when it has fallen 10 degrees.

If the weight of the mercury be thought inconvenient, it may be avoided by the construction described in fig. 2, where the bottom of the tube is bent so as to point upwards, and is joined to a ball A, which communicates with a cylinder placed above it. In all other respects it is the same as the instrument before described. It is filled with spirit of wine and mercury; the quantity of the latter being sufficient to fill the whole tube and ball a. No part of the spirit in the cylinder can get into the tube as long as the instrument is kept in an erect position, or even if it be carefully laid down flat on a table. For though in this last case some of the spirits may get into the ball A, it will rise to that part of the ball which is then uppermost, and will not touch the orifice of the tube n; which was the reason for adding this ball, which would be unnecessary if the instrument was kept constantly erect, or nearly so. If the spirit should come to touch the orifice of the tube n, it would work up between the mercury and the glass; which would put the instrument out of order.

The thermometer fig. 3 is designed for showing the greatest cold which happens in any place during the time the instrument is left in it. The tube is bent into the shape of a syphon of unequal legs standing parallel to one another, the bend being at the bottom. The top of the shorter leg is bent to a right angle, and immediately opens into a ball A, which, by means of a short bent tube on the opposite side, communicates with a cylinder standing parallel to the legs of the syphon, and pointing downwards. This cylinder contains the greatest part of the fluid; and is added only to make the thermometer more sensible than it would be, if the ball A was made of a sufficient size to contain the proper quantity of fluid. This instrument is filled with spirit of wine, with the addition of as much mercury as is sufficient to fill both legs of the syphon, and about a 4th or 5th part of the ball A. The common degrees of heat are shown by the top of the mercury in the longest leg, or by the top of the spirit, in case any is left above the mercury. When the mercury in the longest leg sinks by cold,. that in the shorter leg will rise, and will run over into the ball a: from whence it cannot return back when the thermometer rises again, as the surface of the mercury in the ball is below the orifice of the tube n. Therefore the upper part

of it

of the shorter leg will be filled with a column of spirits of a length proportional. to the increase of heat; the bottom of which, by means of a proper scale, will show how much the thermometer has been lower than it then is; which being substracted from the present height, will give the lowest point that it has been at..

If no further contrivance was used, the mercury would fall into the ball ▲ in:

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