Philosophical Transactions/Volume 2/Number 24
PHILOSOPHICAL
TRANSACTIONS.
Munday, April 8. 1667
The Contents.
DIRECTIONS
THough the Art of Navigation, one of the most useful in the
World, be of late vastly improved, yet remain their many things to be known and done, the knowledg and performance whereof, would tend to the accomplishment of it: As the making of exact Mapps of all Coasts, Ports, Harbors, Bayes, Promontories, Islands, with their several Prospects and Bearings; Describing of Tydes, Depths, Currents, and other things considerable in the Seas: Turnings, Passages, Creeks, Sands, Shelves,
Rocks, and other dangers: Nice Observations of the Variations and Dippings of the Needle, in different places, and in the same place, at different times: The Winds, Weather, and Tempers of the Seasons every where: The great Depths, Ground, and Vegetables at the bottom of the Sea: The various Degrees of Saltness of the Sea-water, in several places, and at several Depths at the same place. If besides Astronomical things, to be hereafter lookt into, the following Experiments be carefully made, and Directions observed by as many Ingenious Persons, as have opportunity, it may fairly be hoped, that from multitudes of Experiments and Observations, such Rules may be framed, as may be of inestimable use for Seamen. To which purpose the Royal Society, having some years ago, ordered that Eminent Mathematician Master Rooke, one of their Fellows, and Geometry Professor of Gresham Colledge (since deceased, to the great detriment of the Common-wealth of Learning) to draw up some Directions for Seamen, the better to capacitate them for making such Observations abroad, as might be pertinent and suitable to the purposes above-mentioned; such Directions were drawn up accordingly, and soon after printed in Num. 8. of these Transactions. But, further to incourage and facilitate the Work of those, that shall be engaged to put them into practice, it was thought fit, that what of this Kind was heretofore but barely proposed, should now be publisht with ample and particular Explanations, and considerable Additions; which done, a good number of such printed Copies is, by the Care, and at the Expences of the R. Society, to be lodged with the Master of Trinity-house, to be recommended to such, as are bound for far Sea-Voyages, and shall be judged fit for the performance: who are also to be desired, to keep an exact Diary of such observations and Experiments, and deliver at their return a fair Copy thereof to the Lord High Admiral of England, his Royal Highness the Duke of York, and, another to Trinity-house, to be perused by the said R. Society.
The Particulars themselves follow:
At Land, where by the help of good fixt Dials, and other fit Instruments, the precise Meridian of the place may be known, it is easie to find the Variation of the Needle, divers ways: As, by applying of the Needle, &c. to the Shadow of a Thread hanging perpendicular when the Sun is in the Meridian; or to the Meridian Line; or the Side of a fixt Horizontal Dial, &c.
But at Sea, in regard the Meridian is not so easie to be found to any tolerable exactness, to know the Variation of the Needle, is much more laborious and difficult. The Height of the Pole, and the Suns Declination being known; a large Ring-Dial, truly wrought, having a Box with a Compass or Needle fixt to its Meridian below, may go as near as any other Instrument, to shew the Variation required. For, when it is let to the just hour and minute of the day, the Meridian of it stands just in its due place; and so shews how far the Needle varies from it, as exactly as the largeness of the Card will permit.
But because these Dials are so rarely just, &c. though they may be used and taken notice of, yet are they not to be relied on. The thing therefore is to be performed, as followeth:
Find out the Suns Azimuthal Distance from the Meridian some hours before, or after Noon, and then its Magnetical Azimuth, or Distance from the Meridian pointed at by the Needle, and the Difference of these two Distances, is the Variation of the Needle.
To find the Suns true Azimuth, or by how many Degrees, &c. of the Horizon it is distant from the Meridian: its Declination, its Altitude, and the Elevation of the Pole, must all three be known. For finding whereof, every Expert Mariner is instructed, or may be so, from his Sea-Books, and so it needs not here to be set down. Nor how by the help of these, the Azimuth required may, to Degrees, if not nearer, be found out upon a good Globe or Planisphere, (whereof there is a design to have one, that is, the Analemma, contrived into a form of Instrument for the use of the publick, and that ere long; which will with great facility perform all that the Globe can do, with much more exactness and conveniency) that being sufficiently known.
But to do it accurately, you must constitute a Spherical Oblique-angled Triangle, of the three Complements, of the Suns Declination, its Altitude, and of the Height of the pole; the measures of all the Sides whereof are known; One from the Zenith to the Pole; another from the Pole to the Point of the Suns Altitude; and the third, from that point to the Zenith, Now by those you are to find out the Angle at the Zeinth, which being found, substract it from 180, and the remainder is the Suns true Azimuth, or Distance from the Meridian of the Place.
This Angle is to be found divers wayes, as by the Tables of Sines, Logarithmes, &c. the manners of doing whereof, are set down and demonstrated by John Newton in his Institutio Mathematica, Case 11. and in other Books of Trigonometry.
And the true Azimuth of the Sun being thus found, and the Magnetical Azimuth of it, according to your Needle, observed, substract the lesser Number from the greater, and the Remainder is the Variation of the Needle. If the Magnetical Azimuth be less than the other, then the Variation is towards the same side of the Meridian, where the Sun is; if greater, on the other.
To observe the Suns Azimuth by the Needle, and the Needles Variation to Degrees, any Needle, long enough to afford upon a Card under it, a Circle divided into Degrees, put in a Square Box, after the ordinary manner of Clinatories, will serve turn; by placing the Box so, as the Sun may shine upon any two opposite sides of it, at the same times that the Suns Height, &c. are taken: For then the Needles Distance from the Diameter of the Circle on the Card, that is parallel to those sides, is the Magnetical Azimuth required.
The same may be done with an ordinary Sea Compass, so it have a Circle towards the Limb of the Card, divided into Degrees, by fastening a small Thread, Lute-string or Wire (not of Iron) so upon it, as to pass just over the Center of that Circle; and placing a strait piece of wood or Brass-wire perpendicular on the edge of the Box at the end of the Thread, and turning it to the Sun, till the Shadow of it fall just upon the Thread: then observe, what Degree of the Circle on the Card the Thread cuts, by looking plum upon it; and that is the Suns Magnetical Azimuth.
But to have the Variation to Degrees and Minutes (which is most definable) then the Observation last mention'd must be made with a Quadrant, Sextant, or some such other Instrument, so large as to admit of the division of a Degree into Minutes; which will require the Radius to be about three foot; the larger the better. If a Quadrant, then, it being laid flat, and the Square Box with the Needle placed upon it, move the Quadrant to and again, till that side of it, on which the Box is placed, lie parallel to the Needle, when at quiet: Then the Sight of the Quadrant being slid along the Rimb of it, till the Sun shine on both its sides at the same time, the Mid-Line, that divides equally the Sight, when the Sun shines upon it through the slit, will mark the Degree. and Minute of the Suns Magnetical Azimuth. All which is easie to be put in practice.
And if many such Observations be made by Several persons at the same place; and by the same or other persons distant from one another, 1.10.20. or more Years; Not only will the Compass become more useful then formerly, even to be conducive possibly to the finding the Lougitude at Sea, at least in some places: but the variation of the Variation of the Needle being known in different places, all will be reduced to Rules, and so from hence, Philosophical or Natural Knowledg, will probably be enlarged by a happy discovery of the true cause of the Verticity, or Directive faculty of the Loadstone; one of the Noblest and most abstruce Phænomena, that falls under the cognizance of humane Reason.
To find this variation by the Stars, is so easie, as every Master can do it; seeing there is no more requisite, than to find out the true North, that is the Meridian, and compare the Needles position with it. By this means, the variation may be had well enough to degrees, half degrees, and some smaller parts; and if carefully and curiously prosecuted, even to Minutes too. But it will not be amiss, to do it both by the Sun and Stars, that the greater certainty may be attained.
The Dipping-Needle is to be used at least as Frequently as the former Experiment is made, and in the same places, in order to the same purposes. All that needs be said of the Manner, is, that when the Dipping of the Needle is to be examined, the Circle, in which it moves, is to be hung perpendicular, and turned, till it be just in the Magnetical Meridian, where it dippeth most, and the degree of its depression under the Horizon is to be noted in a Table. See Figure 1.
The Particulars here to be regarded, are, 1. The precise times of the beginnings of the Flood and Ebb, in all Rivers, Bayes, at Promontories, Capes, and in all Roads, Harbours, &c. 2. Which way Currents run in all places, with their Times, Changes, &c. 3. What perpendicular Distance there is, between the highest reach of the Tide, and lowest of the Ebb, both of all Spring-Tides and Neap-Tides, with their irregularities, &c. 4. What day of the Moon's age, and what times of the Year the highest and lowest Tides fall out: And all other considerable Accidents observable in Tides, chiefly in and near all Sea-ports, Harbours, Roads, Islands, &c. as St. Helens Island, Bermuda. 5. The position of the Wind at every Observation of the Tides, &c.
To perform this, take a Globe of Firr, or Maple, or other light
wood, as A, let it be well secured by Vernish, Pitch, or otherwise,
from imbibing Water, then take a piece of Lead or stone,
D, considerably heavier than will sink the Globe: Let there be a long Wine-staple B in the Ball A, and a springing wire C, with a
bended end F, and into the said Staple, press in with your fingers
the springing Wire on the bended end: and on it hang the
weight D, by its hook E, and so let Globe and all sink gently
into the water, in the posture represented in the said Figure, to the
bottom, where the weight D touching first, is thereby stopt; but
the Ball, being by the Impetus it acquired in descending, carried
downwards a little after the weight is stopt, suffers the springing
Wire to fly back, and thereby sets it self at liberty to re-ascend.
And by observing the time of the Ball's stay under water (which
may be done by a Watch, having Minutes and Seconds; or by a
good Minute-Glass; or best of all, by a Pendulum, vibrating Seconds;
the which must be three foot, three inches, and one fifth
of an inch long, viz. between the middle of the Bullet and the
upper end of the Thread, where it is fastned, or held when it
vibrates.) You may by this way, with the help of some Tables,
come to know any depth of the Sea.
Note, That care must be had of proportioning the weight and shape of the Lead, to the bulk, weight, and figure of the Globe, after such a manner, as upon experience shall be found most convenient.
In some of the Trials already made 'with this Instrument, the Globe being of Maple-wood, well covered with Pitch, to hinder soaking in, was 5 inches in Diameter, and weighed 2½ pounds; the Lead, of 4½ pounds weight, was of a Conical (but is now used of a Globous) Figure 11 inches long, with the sharper end downwards, 1 at the bottom in Diameter. And, in those Experiments made in the Thames, in the depth of 19 foot water, there passed between the Immersion and Emersion of the Globe, 6 Seconds of an hour; and in the depth of 10 foot water, there passed 3½, Seconds, or thereabouts: From many of which kind of Experiments, it will likely not be hard to find out a method to calculate, what depth is to be concluded from any time of the like Globes stay under water: As for instance, if in the depth of 20 fathom, measured by the Line, the Globe stay under water 15 Seconds; then if the Ball stay 600 Seconds, the depth of the Sea is 933 fathom and 2 foot, if the Ball be found to move equal spaces in equal time.
In the same Trials made with this Instrument in the said River of Thames, it has been found, that there was no difference in time, between the submersions of the Ball at the greatest depth, when it rose two Wherry's length from the place where it was let fall (being carried by the Current of the Tyde) and when it rose onely a Yard, or so, from the same place, where it was let down: And that it must be so in great depths and stronger Currents, is as certain, as easie to be demonstrated.
And if it be alledged, that it must be known, when a Light Body ascends from the bottom of the Water to the Top, in what proportion of time it rises; it may be considered, that in this Experiment the times of the Descent and Ascent are both taken and computed together; so that, for this purpose, there needs not the nicety, which is alledged.
Of other Experiments of this way of sounding without a Line, made by the Noble Lord Viscount Brounker, Sir Robert Moray Knight, and Mr. Hook, in the Channel at Sheerness; the following account was given, Vid.
| Weighed. | Ounce. | Grains. |
| A Wooden Ball (A) | 32 | 00 |
| Another, Wooden Ball (B) | 30 | 22 |
| A Lead (A) | 30 | 00 |
| Another Lead (B) | 30¾ | 00 |
The Ball (B) and the Lead (B) were let down at 16 fathom 5 and the Ball returned in 48 single strokes of a Pendulum, held in the hand, vibrating 58 single strokes in a Minute.
A second time repeated with the same success; therefore, the motion was 4 foot every second.
Again the Ball (A) and the Lead (B) whose Nail was bended into a sharper Angle; the Ball returned in 39 strokes. A second time repeated with the same success at the same depth.
Ball (B) Lead (B:) in which trial the Line, not being clear, stopped a little the motion; the Ball returned in 47 at the same depth.
Ball (A) Lead (A) at 8 fathom and 1 foot, returned at 20; repeated at 8 fathom, returned at 19.
Tried the third time at 10 fathom and 4 foot, return'd at 28.
A fourth Trial, at the same depth, just the same.
A fifth, at 10 fathom, 5 foot, returned in 27.
A sixth Trial, just the same.
A seventh, at 12 fathom, 5 Foot returned in 37.
An eighth Trial, just the same,
Another Day, near the same place.
Note, That the Pendulum was this Day adjusted, and made a little shorter, there having been but 58 vibrations in a Minute, the other day.
Ball (A) Lead (B) at 14 fathom, returned in 32½.
A second Trial, a little after in the same place, returned in 33. In making of which Trial, the Vibrations were told aloud, and the Lead having been let down by a Line, was found to touch the bottom in just half the time, the Ball staid under water. By a second Trial, the ascending and descending was found to be in equal times. And by a third Trial with another Lead, the very same found, vid. 16½ descending, and 16½ ascending. This Lead and Ball let down without a Line, the Ball returned in 13 vibrations; a sign it went not to the bottom.
the top of it (like that in the Figure 3) succeeded very well, and the Ball returned in 34½: But by reason of the Current, the Experimenters could not perceive, when the Lead touched the bottom. This Lead being let down without a Line, the Ball returned in 32½. The depth of the water was now found by the Ships Lead, to be 14 fathom.
the Lead (like that in the Fig. 4) and the Ball return'd in 34. The same let down without a Line, the Ball return'd in 6 or 7 vibrations; a sign again, it went not to the bottom.
In a Trial with another Lead, the Ball return'd in 34.
Repeated again with the same success.
of the Fig 5.) the Ball returned in 34. After which Trial the depth was found to be just 14 fathom.
The last Lead and Ball being let down without a Line, the Ball returned at 35.
In another Trial with a Lead that never failed, the Ball returned in 34, and the Lead toucht the bottom at 17.
By a Trial with another Lead, the same time was found exactly.
By a third Trial with this last, the very same.
These Trials were made near about High-water, at the depth of 14 fathom just by measure: And in them, the motions seem to be 5 foot every second.
In all these Trials, the greatest difficulty was, in the use of Conical Figures, with Iron Crooks, to bend the Iron, that it might be sure to carry down the Bill with it to the bottom, and when come thither, to let it go: for almost every one of these Leads failed in one of these requisites, till by several Trials they had been adjusted.
It is not to be omitted, That the last Trials being made near High-water, the Ball was found to rite (by the Boat, being permitted to drive) far off upon one side, out of the way, that any light thing, suffered to swim on the water, would be carried; which seemed to argue a motion of the under parts of the water, differing from that of the upper (a thing which is said to be at certain times of the Tydes, both at the Mouth of the Sound, and of the Streights; which deserves to be further inquired into.) The
Angle made by these different motions, seemed to be about 40 Degrees. 
such is represented by Figure 6; which being exposed to the Wind, so as the flat side may be right against it, the number of Degrees upon the Limb A B, to which the Wind blows-up, or raises, that flat side C D, shews the force or strength of the Wind, in proportion to the resistance of the flat side of the Instrument; and is to be recorded.
| The Form of a Scheme. | |||||||||
| Which at one view represents to the Eye Observations of the Weather, for a whole Month, may be such, as follows. | |||||||||
Days of the Moneth, and Place of the Sun. |
Remarkable hours. |
Age and sign of the Moon at Noon. |
The Quarters of the Wind, and its strength. |
The Faces or visible appearances of the Sky.
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The Notablest Effects | General Deductions These are to be made after the side is filled with Obsverations, as | |||
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4 |
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Clear blue, but yellowish in the N.E. | A great Dew. | From the last Quarter of the Moon to the Change, the weather was very temperate, but for the Season, cold; the Wind pretty constant between N. and W.
&c. | ||||
| June | 
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27 | |||||||
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8 |
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Clouded toward the South.
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| 14 |
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Thunder far to the S.
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12 | ȣ 9.46 |
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| II | 
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4 |
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Checkered blue. | ||||||
| 12.46' |
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Perigeum | A very geat Tyde.
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8 | WSW 1 | |||||||
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12 |
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8 | NW 3 | A clear sky all day, but a little checker'd about 4 P. M. At Sun-set red and hazy. | Not by much so big a Tyde as yesterday. A great Thunder-Showre from the N. | |||||
| 15 |
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28 | |||||||
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4 | 4 | |||||||
| II |
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6 | ȣ 24.51 | N 2 | ||||||
| 13.40' |
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12 | 1 | |||||||
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10 | New Moon at 7. 25. A. M. | Overcast and very lowring, &c.
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No dew upon the ground, but very much upon Marble-stones, &c.
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| 16 |
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S 1 | |||||||
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| II |
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II 10.8 | &c. | ||||||
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| 14.57 |
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&c. | |||||||
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The Viol is to be made with a very narrow Neck, and when it is almost full, water is to be dropt into it, drop by drop, till it can hold no more, drying well the Viol before it be weiged. The weight of the empty Viol is also to be recorded every time, weighing all to grains. And by evaporating gently the water, till the Salt be left dry on the bottom; they, who list, may have the satisfaction to know, what proportion the Salt of each water holdeth to its weight.
There is, among some other ways of finding the different gravities of Water, a very pretty one, mentioned by some Authors, as Johannes Toldenus (a German Artist) Cabæus, and Kircher in his Fundus subterram, and improved and first brought into use here, divers years agoe, by the Noble R. Boyle, who also, as himself informed the Publisher, hath in some of his Writings, yet unpublisht, set down a full Description thereof.
It is such a Glass-Tube as is represented by Fig. 7. blown at a Lamp, and poised in good common Water by putting Quick-silver into it, until it sink so low, that nothing appear above the Superficies of the Water, but the Top; which done it is to be sealed up, and to be graduated on its side, into what parts you please; which may be done with a Diamond. And then, being put into any Water to be weighed, it will, by its more or less sinking into it, shew the differences of the Waters gravity.
To perform this, let there be made a Square Wooden Bucket (such as C in Fig. 8.) whole bottoms E E are to be so contrived, that the weight A do sink the Iron B (to which the Bucket C is fastned by two Handles D D, on the ends of which are the moveable bottoms or Valves E E) and thereby draws down the Bucket, the resistance of the Water keeps up the Bucket in the posture C, whereby the Water hath-a-clear thorow-passage all the while it is descending; whereas as soon as the Bucket is pulled upwards by the Line F, the resistance of the Water to that motion, beats the Bucket downward, and keeps it in the posture G; whereby the included Water is preserved from going out, and the Ambient Water kept from getting in.
By the advantage of which Vessel, or such like, you may come to know the Degrees of Saltness of Sea-Water, according to its nearness to the Top or Bottom; or rather, the Constitution of the Sea-Water in several Depths of several Climates: Likewise, whether in some places of the Sea, there be any sweet Water at the Bottom; the Affirmative whereof is to be met with in the East-Indian Voyages of Van Linschoten, who pag. 16 of that Work, as 'tis Englished, records, that in the Persian Gulf, about the Isle of Baharem, fetch up with certain Vessels (which he describes not) Water out of the Sea, from under the Salt-Water, four or five fathom deep, as sweet as any Fountain-water, And since 'tis argued by some, that such Sweet-water proceeds from certain Sweet Water-Springs, that were formerly on the Continent, at some distance from the Sea, and came afterwards to be covered by the Sea; it may be presumed, that in other places we may find the like. Besides, we know not, but that there may be in many parts, Eruptions of large Springs at the Bottom of the Sea, that were never taken in by any of its encroachments.
These Experiments are to be repeated every New Voyage, the multitude and frequency of them being necessary for finding out and confirming the truth of them; which as it will conduce exceedingly to the Enlargement of Natural Knowledge, so it may in time produce New and more accurate Sea-Maps and Cards, than hitherto have been publisht; and great helps and advantages to Navigation: especially those of the Variation, and Dipping of the Needle; the Depth and Saltness of the Water, the Nature of the Ground at the Bottom of the Sea; and indeed almost every one of the rest; there being a Design to consider all, and to draw out of them such Rules and Directions, as may bring no less Honour, than Benefit to the English Nation.
The Instruments, described and represented in these Papers, may be had from Mr. Richard Shorgrave, Operator to the R. Society, to be found at Gresham Colledge; who also will be ready, if there be occasion, to give more particular Directions for the use of the same.
Advertisement.
It is desired by Christopher Merret M. D. to inform the Publick, that within the space of four Months, he shall re-publish his Pinax Rerum Naturalium Britanicarum, with many additions, and in his proposed New Method; and that he wholly disclaims the Second Edition of that Book, as being printed and published without his knowledge.
In the SAVOY,
Printed by T.N. and John Martin at the Bell, a little without Temple-Bar, for James: Allestry in Duck-Lane, Printers to the Royal Society, 1667.