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Westerly foro Variation
Merid of O Greenwich
holes be successively plugged up. When the Ship foats at her load
known Neov quantit
water-line, she will contain a quantity of water equal to her internal figiae capacity, which is her tonnage : then let all the salt or fresh water she
tontains be pumped out into tubs, containing half a ton each ; and
when all the water is pumped out, her exact tonnage will be known, Wayda as also her best sailing trim in every gradation of her immersion, from
her light water-line, to her load water-line.
N. B. As salt and fresh water differ in weight, the tubs must be
proportioned accordingly. N ( Thirdly,—Take a quantity of iron ballast whose weight is known,
and load the Ship with it to her load water-line, which will give her internal capacity. That calculation is best which comes nearest to the tonnage given by these methods.
N. B. The above is intended as a general treatise on the property of Ships ; but should any person be desirous of obtaining accurate information respecting Ship-building, I beg leave to refer them to the regular publications of the Society FOR THE ENCOURAGEMENT OF NAYAL ARCHITECTURE.
L. [To be continued occasionally.] F
CHART OF THE LINES OF MAGNETIC VARIATION
SEAS AROUND AFRICA. [Copied by Permission from Major Rennell's Appendix to Mr. Park's Travels.]
GREAT part of Mr. Park's geographical memorandums 'are
totally lost ; but fortunately his bearings by Compass * during a great part of the way, are preserved. In other parts he has preserved only the calculation of latitude and longitude, arising from them ; which, however, of course furnish the means of obtaining the bearings, if necessary. As he omitted to take observations to determine the quantity of the variation of the Compass, after he lost the means of correcting his course by observations of latitude, which was at Jarra, about midway in his route + : it becomes a question of some importance, what quantity to allow on those lung lines of distance between Jarra and Silla ; Silla and Manding.
It appears on inquiry, that the quantity of variation is no more known any where within the continent of Africa, than within that of New Holland. And it happens moreover, that the lines of equal quantities of variation, do not run across Africa with that degree of
Appendix to Mr. Park's Travels, page 26. + The places of observation are marked on the map by asterisks.
regularity and parallelism, which takes place over great part of the Atlantic and Indian Oceans fat least this is what appears clearly to my judgment): so that it became necessary to enquire, what quantity prevails in the surrounding seas; and what the general direction, as well as the particular nature, and tendency, of the curves, of the lines of equal quantities.
The variation lines on the globe have occupied a good deal of my attention, at different periods of my life ; and therefore the application of such new observations, as the assiduity and kindness of nry friends had procured for me on this occasion, was Jess difficult, than if the subject had been new to me. A dissertation on the subject, would be out of place here ; and therefore I shall only give the result of my inquiries, in abstract; after premising, that the theoretical part belonging to the interior of Africa, is founded on a supposed continuation of those lines of equal quantities; whose teridency has been already ascertained, in the surrounding seas, I am perfeclly aware, that some may regard the assumption as too great : but they will no doubt admit, at the same time, that it is difficult to conceive a more pro. bable arrangement : and what is much more to the purpose, is, that if we are compelled to abandon the system, in the gross, the quantity of variation in the line of Mr. Park's travels, cannot be greatly different from what we have assumed. For, whether the line of 18 degrees in the south Atlantic, be a continuation of that in the north Atlantic, or of that in the Fadian Sea, much the same result will follow : only that in the former case, the quantity will be some. what greater.
It would appear, that between the East Indies and South Ame. rica, Europe and South Africa, there are four distinct sets of what may be termed concentric curves of variation lines, on the globe; and whose highest points of convexity are opposed to cach other, withio the great body of Northern Africa. The accompanying sketch will best explain it *. It would appear moreover, that from the place of
• This sketch is not pretended to be minutely accurate; it being morally is possible to procure recent observations in every part, from the rapid change that takes place in the quantity of the variation, in one and the same spot. However, the observations that determine the course of the lines in the Atlantic (and which are marked on the sketch) are from observations so late as 1793. The same is to be said of those in the western quarter of the Meditcrranean ; and those beyond the Cape of Good Hope, to longitude 30 degrees east, are of the year 1789
It is obviors that a critical knowledge of the quantity of tħe variation in any particular place, and at a given time, is of less importance to the present queszion, than that of the bearirg of the lines of cqual quantities, at any recent period; and this object is, I think, tolerably well obtained by the materials before me. Any change that may have taken place since 1793, is in favour of a greator quantity of variacion, within the limits of Mr. Park's travels,