Part 20
If the Doctor will immerse some of these corallines, when they are extended, in two thirds of spirit of wine and one third of clear sea-water, it will preserve them many years, as I have experienced. He may then put the different sorts into distinct phials, and view them at pleasure with a lens of about one inch and half focus.
In fine, my Lord, opportunities so seldom offer at the sea-side to make these experiments with accuracy; and likewise to this, the strong lines of vegetation that these bodies carry in their appearance, and your Lordship will not be surprised, that there are so many gentlemen, even of the Royal Society, that totally disbelieve them to be animals.
Many there are in the Society, that are wavering between both opinions. If then, my Lord, you think, that any specimens which I have, or any demonstrations tending to clear up this point, that lie in my power, will be acceptable to your Lordship and the Society, your Lordship may freely command them, whenever you think proper, from
Your +LORDSHIP’S+ Much obliged and most obedient Servant, John Ellis.
London, June 9. 1757.
XXXIV. _An Account of an extraordinary Operation performed in the Dock-Yard at_ Portsmouth: _Drawn up by Mr._ John Robertson, _F.R.S._
[Read May 26, 1757.]
THE Royal William, a first rate man of war, built about 40 years ago, having, upon examination, been judged in so good a state, as to be worthy of repairing for sea service, was ordered into dock, and brought thither on the 29th of June 1756. On these occasions it is usual to lay across the middle line of the bottom of the dock, at distances of about five feet from one another, thick pieces of oak timber of about four feet long; their upper surfaces lying in the same plane, or so posited, that a line stretched from the two extreme blocks will touch all the intermediate ones; and on the middle of these blocks the keel of the ship is to rest. On the said day the tide did not rise so high as was expected; and there was not quite depth enough of water to float the ship in, and set her on the blocks, notwithstanding the assistance of an empty lighter, which, being fixed to the stern, lifted the ship at the end six inches: and as the officers knew they should not have so much water again before the next spring-tides, they were determined to heave her in; which is a very common operation in most dock-yards. Now it so happened, thro’ the great weight of the head and stern, that the ship cambered very much; that is, her keel, from being straight, was become much curved, the two extremities hanging lower than the middle part by many inches; and consequently the foremost part of the keel, instead of sliding over the blocks, forced all the foremost ones away, for above 60 feet; whereby that part of the keel rested on the bottom or floor of the dock, and the aftermost part rested on such of the blocks, as had escaped the violence, which had displaced the others. In this situation the keel was very far from being strait; and so it was resolved to lift by main force the head of the ship, until the keel should be strait; and in that position to support it by the blocks, which had been forced away from their places.
For this purpose there were set up, under the wales and other parts of the ship, to the length of near 80 feet of the stem, as many shoars, as were judged necessary; and also nine pair of bed-screws, three pair under each bow, and three pair under the knee of the head. At each shoar a workman was appointed, to drive wedges between the heels of the shoars and the parts of the dock whereon they rested; whereby the shoars were raised end-wise, and consequently the body of the ship lifted at the same time. While this was doing, the 18 screws were also at work: and between these efforts the fore part of the ship was raised upwards of 19 inches, so much being necessary to bring the fore part of the keel in a right line with the hinder part.
In this service were employed about 270 men; whereof about 144 worked at the screws, and the others worked at the shoars with their mawls and wedges; and the whole operation was performed in about seven hours.
My curiosity leading me to inquire what was the weight of the ship, in the condition she was at the time of bringing her into the dock; for this purpose I procured draughts of the elevation and section, and of the plans at the line of floating, and at the parallel sections of every foot distance down to the keel. Then, by finding the mean area between every two sections, I was thereby enabled to come at the magnitude of a solid, that would nearly fill the trough the ship made in the water; and, by increasing this magnitude by that of the keel, and so much of the stern-post and stem, as were under water, the cubic feet of the fluid displaced by the ship were obtained, being 54869; and consequently her weight was 3532091 pounds, or 1576 tons, 16 _C_. 2 qrs. 3 ℔. These numbers were not altogether so easily come at, as they would have been, had the ship swam on an even keel, her draught of water before being 13 feet 2 inches, and abast 16 feet 6 inches. However, the computation may be esteemed as correct as the nature of the subject would admit; because I found pretty near the same solidity by another method.
I got a block or model made, by a scale of a quarter of an inch to a foot, of so much of the Royal William’s body, as was immerged, when she was brought into dock; and this block I immersed in a trough of sea-water, and found its weight in the following manner.
The length of the trough was 46 inches, breadth 14 inches, and depth 8 inches: at each corner was a graduated scale of inches, and pencil-lines drawn round the inside of the trough at every inch. Sea-water was poured into the trough to the height of 5 inches; and the trough was exactly levelled, by means of the pencil-line, at 5 inches: then the block being forced under the water’s surface, the fluid, when still, was risen to 6⅓ inches; consequently the magnitude of the block was equal to a parallelopipedon of 46 inches long, 14 inches broad, and 1⅓ inches deep, or to 858⅔ cubic inches.
Now 858⅔ cubic inches are equal to 0.4969 cubic feet.
And a cubic foot of sea-water weighs 64.373² pounds avoirdupoize.
Then 64.373² × 0.4969 = 31.987 pounds.
So that by a quarter inch scale, a model similar to the Royal William weighs near 32 ℔.
But a quarter inch scale is ⅟48 of a foot scale.
And the model is to the ship as 1³ is to 48³, or as 1 is to 110592.
Then 3537506 ℔. (= 110592 × 31.987), or 1579 tons, 4 _C._ 3 qrs. 14 ℔. is the weight sought.
The difference by the two methods amounts to 5415 ℔. or to 2 tons, 8 _C._ 1 qr. 11 ℔.
Some of the persons present at this experiment read the height of the water at 6⅜ inches: the difference between 6⅜ and 6⅓ inches is ⅟24 of an inch; a difference easily to be made by different persons in an experiment of this kind. But observing, that the computation made on 6⅜ inches amounted to near 50 tons more than on 6⅓ inches, I caused the trough to be diminished in its depth to 6½ inches, had one of the ends cut off, and a board fixed on the open side, being desirous of making the experiment with the trough standing on one end: and indeed, in this situation, an error of ⅒ of an inch in the height of the water makes a difference of about 16½ tons in the weight of the ship. Into this upright trough water was poured to the height of 36 inches; and the block being immerged, the water was raised 9⅓ inches: so that the block was equal in magnitude to a parallelopipedon of 14 inches long, 6½ inches wide, and 9⅓ inches deep, or to 849⅓ cubic inches: from whence I find the weight of the ship to be 1562 tons, 1 _C._ 2 qrs. 16 ℔. And altho’ I take this number to be nearest the truth, yet it may be observed, that it is no easy matter to come at accuracy in this subject by any of the methods in common use.
My next inquiry was, to find how much of this weight was lifted, and how to proportion it among the screws and mawl-men: but in this, less accuracy must be expected than in the preceding inquiry; for the exact number of men employed is not known; neither can it be told, how many worked at the screws, and how many with the mawls; and only a guess can be made at the part lifted. However, something may be gathered, which may, perhaps, be worth the knowing.
Let the weight raised be taken at half the weight of the ship; for 64 feet, the length of the keel raised, is not far from half the whole length: add to this the sally of the head, the weight of the forecastle, the friction of the timber, and the resistance of the parts bent by the cambering: beside, the mawls worked at several shoars set up abast the said 64 feet.
Now the weight by the last experiment was 3499064 pounds: one half, or 1749532 ℔. I take to be the weight raised between the screws and mawls.
The distance between two contiguous threads of each screw was 1⅓ inches; the length of the two opposite levers was 12 feet 8 inches, or 152 inches, and described a circumference of 477½ inches: each screw was worked by 8 men: their force, reckoned at 30 ℔. each, makes the power working on each screw equal to 240 ℔.
Hence, from the known property, each screw could raise 65485 ℔.
And the 18 screws raised 1178730 ℔.
Then there remained 570802 ℔. to be raised among about 126 mawls:
Which gives 4530 ℔, or a little more than two tons, to be raised by each man with his mawl and wedges; which is considerably less than what I have seen raised by way of experiment.
XXXV. _Observations on an Evening, or rather Nocturnal, Solar_ Iris. _By Mr._ George Edwards, _Librarian of the College of Physicians_.
_To the Reverend Dr._ Birch.
[Read June 16, 1757.]
SIR,
ON Sunday evening the 5th of June 1757, being walking in the fields near Islington, about half a mile north of the upper reservoir or bason of the New River, I 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, as near as I could judge, it then being darkish, I was greatly surprised to see 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, tho’ 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, until the whole arch disappeared. The center of the arch was above the horizon at its first appearance. What most perplexed me, was, to find the cause of this painted arch. I could not believe, that it 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, and somewhat hazy in the quarter where the bow appeared; which was not near so bright as the rainbow appears to be in the day-time; and I believe, that it would not have been visible at all in the presence of the sun. I imagine it was formed on the gross particles of the evening vapours, mixed with those of the smoke arising from the town; for had the sun-beams shot from beneath the horizon on falling rain at a considerable height above the earth, I believe the darkness would have rendered the appearance of such a bow far brighter than it appears to the sight in the presence of the sun: but this night or evening arch being reflected, as I suppose, from particles so minute as those of floating vapours, gave but little light and colour to the sight, and what would not have been visible, had the sun been above the horizon. For the same reason, the moon and stars are visible in the absence of the sun, and, on the contrary, are unseen when the sun is present: and if we light a candle, and set it in the sun-beams, the flame is lost to our sight, tho’ the same candle will give us a considerable share of light in the night. As I have never before seen or heard of such an arch, I thought this account of it (imperfect as it is) might not be disagreeable to the Royal Society.
It 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. The consciousness of my inability to give a proper account of such an uncommon appearance could not deter me from the attempt.
I think I have said all that is necessary on this subject; yet am ready to answer any question for the farther illustrating of it. I am,
Reverend Sir, Your most humble Servant, Geo. Edwards.
College of Physicians, London, June 6th, 1757.
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._
[Read June 23, 1757.]
June 23d, 1757.
MR. Baker received a letter yesterday from Dr. Alexander Garden, of Charles Town in South Carolina, part of which he hopes he shall be excused for laying before the Royal Society.
The Doctor writes thus:----“As you desired, I tried the effects of the prickly pear in colouring the urine. A few days after your letter, I went down to one of the islands, and gathered some of the fruit, and gave four of the pears to a child of three years of age, and six pears to one of five. The next morning I examined the urine of both, and it 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 I gave six pears to a Negroe wench, who gave suck, and strictly forbad her suckling her child for six or eight 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, tho’ it was very faint.
I was led to this last experiment by an observation, which I made on the milk of cows, who 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.”
Dr. Garden wrote, a year ago, that the prickly pear grows in great abundance about Carolina; and also that the cochineal insects are found upon it; but hitherto no attempts have been made to cure them as the Spaniards do. In hope, that some rich dye may be produced from the plant itself, Mr. Baker proposed some experiments to Dr. Garden, which he intends to prosecute this summer.
XXXVII. _Account of an extraordinary Shower of black Dust, that fell in the Island of_ Zetland _20th_ October 1755[194]. _In a Letter from Sir_ Andrew Mitchell, _of_ Westshore, _Bart. to_ John Pringle, _M.D. F.R.S._
[Read June 23, 1757.]
Pall-Mall, June 9th, 1757. SIR,
IN compliance with your desire, I made particular inquiry, whether at or about the time the earthquake happened at Lisbon the 1st of November 1755. any uncommon phænomena were observed to appear in the islands of Orkney or Zetland, as such had happened about that time in other parts of Scotland. From Orkney I was informed, that nothing particular had happened; only, that about the time mentioned the tides were observed to be much higher than ordinary. I 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; wherein he writes verbatim as follows. “Blessed be God, notwithstanding the great devastations, that have been made in other parts of the world by earthquakes, we have been intirely free from any disaster of that nature: nor has any thing extraordinary happened in this country since you left it; only on Monday the 20th October last, betwixt the hours of three and four 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, tho’ in greater quantities in some places than in others. It was very much like lampblack; but smelled 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. But I shall trouble you no more about it, as no doubt some of your friends have written to you of it some time ago.”----
In June 1756. I returned to Zetland; and, upon further inquiry, found what Mr. Brown had written me was attested by Mr. Mitchell, parson of the parish of Tengwall, and by several Gentlemen of credit and reputation, who had seen and observed the same phænomenon in different parts of the country at the time above-mentioned.
Mr. Brown having omitted to mention, how the wind did blow at the time the black dust was observed, I made particular inquiry about that circumstance, and found it 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. But, in this case, would not this black dust have been observed in Zetland at its first travelling to the southward? Upon inquiry, I did not hear it was.
Thus far I have obeyed your commands, which I will always do with pleasure; and if you think it worth while to lay this letter before the Royal Society, I leave you at full liberty to do so, or not, as you think proper: but what it contains may be relied on as truth. I am, with great regard,
Dear Sir, Your most obedient humble Servant, And. Mitchell.
_P.S._ I may add, that the distance from mount Hecla to Zetland is between 500 and 600 miles.
XXXVIII. _A Description of some Thermometers for particular Uses. By the Right Honourable the Lord_ Charles Cavendish, _V.P.R.S._
[Read June 30, 1757.]
THE thermometer (TAB. XI. _fig._ 1.) is designed for shewing the greatest degree of heat, which happens in any place during the absence of the observer. It consists 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 shews 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 shew 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 tube, 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 will be 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 shews 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 to say, 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 sixtieth part: and the error arising from making use of the same scale will be about one sixth of a degree, if the thermometer is observed when it has fallen ten degrees.
In the instrument here described, the bore of the tube is about 0.027 inches; and one inch of it contains two grains of mercury, and answers to about ten degrees, the cylinder containing about 2280 grains. If a much shorter tube was made use of, a considerable error might arise from too great a quantity of spirits adhering to the sides of the tube, in that part, which is filled with mercury; especially when the thermometer rises fast. This makes it necessary to employ a cylinder of a considerable bigness, if it is desired to have the scale of degrees pretty large.
If the weight of the mercury is 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 the ball A.
No part of the spirit, with which the cylinder is filled, can get into the tube, as long as the instrument is kept in an erect position, or even if it is carefully laid down flat on a table. For tho’ 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.