Supposing therefore such a Circulation, as above, 'tis to be considered that to the Northward of the Indian Ocean there is every where Land within the usual limit of the Latitude of 30, viz. Arabia, Persia, India, &c. which for the same reason as the Mediterranean Parts of Africa, are subject to unsufferable Heats when the Sun is to the North, passing nearly Vertical; but yet are temperate enough when the Sun is removed towards the other Tropick; because of a ridge of Mountains at some distance within the Land, said to be frequently in Winter cover'd with Snow, over which the Air, as it passes, must needs be much chill'd. Hence it comes to pass, that the Air coming according to the general Rule, out of the N. E. in the Indian Seas, is sometimes hotter, sometimes colder, than that which by this Circulation is return'd out of the S. W. and by consequence, sometimes the under Current or Wind, is from the N. E. sometimes from the S. W.
That this has no other Cause, is clear from the times wherein these Winds set in, viz. in April, when the Sun begins to warm those Countries to the North, the S. W. Monsoon begins, and blows during the Heats till October; when the Sun being retir'd, and all things growing cooler Northward, and the Heat increasing to the South, the North-East Winds enter and blow all the Winter till April again. And it is undoubtedly from the same Principle that to the Southwards of the Æquator, in part of the Indian Ocean, the North-West Winds succeed to the South-East, when the Sun draws near the Tropick of Capricorn; but I must confess, that in this latter occurs a difficulty, not well to be accounted for, which is, why this Change of the Monsoons should be any more in this Ocean, than in the same Latitudes in the Æthopick, where there is nothing more certain than a S. E. Wind all the Year.
'Tis likewise very hard to conceive why the limits of the Trade-wind should be fixt, about the thirtieth Degree of Latitude all round the Globe; and that they should so seldom transgress or fall short of those bounds; as also that in the Indian Sea, only the Northern Part should be subject to the changeable Monsoons, and in the Southern there be a constant S. E.
These are Particulars that merit to be consider'd more at large, and furnish a sufficient Subject for a just Volume, which will be a very commendable Task for such, who being us'd to Philosophick Contemplation, shall have leisure to apply their serious Thoughts about it.
Plate 2 pag. 80
A new & Correct Sea chart of the whole World Shewing the Variations of ye COMPASS as they were found Año 1700 with a View of the Generall and Coasting Trade Winds and Monsoons or shifting Trade Winds by the Direction of Capt. Edm. Halley.
A Discourse of the Rule of the Decrease of the height of the Mercury in the Barometer, according as Places are elevated above the Surface of the Earth; with an Attempt to discover the true Reason of the Rising and Falling of the Mercury, upon Change of Weather. By Edm. Halley.
THE Elastick Property of the Air has been long since made out, by Experiments before the Royal Society, and elsewhere; and the Resistance of its Spring is found to be nearly equal to the Weight or Force that compresses it; as also, that the Spaces the same Air occupies, under differing Pressures, are reciprocally as those Pressures: It has been shewn likewise by undoubted Experiment, that the specifick Gravity of the Air, near the Earth's Surface to that of Water, was once as 1 to 840; again as 1 to 852; and a third time, in a very large Vessel holding 10 Gallons, as 1 to 860; all which, considering the Difficulty of the Experiment, agree well enough, the Mercury standing at all those times about 29 Inches ¾: But by Reason 'twas Summer-weather, and consequently the Air rarified, when all these were tried, we may without sensible Error say in round numbers, that the Barometer standing at 30 Inches, and in a mean State of Heat and Cold, the specifick Gravity of the Air to Water, is as 1 to 800. By the like Trials the weight of Mercury to Water, is as 13½ to 1, or very near it; so that the weight of Mercury to Air, is as 10800 to 1; and a Cylinder of Air of 10800 Inches or 900 Feet, is equal to an Inch of Mercury; and were the Air of an equal density like Water, the whole Atmosphere would be no more than 5,1 Miles high, and in the Ascent of every 900 Feet the Barometer would sink an Inch. But the Expansion of the Air increasing in the same proportion as the incumbent weight of the Atmosphere decreases; that is, as the Mercury in the Barometer sinks; the upper Parts of the Air are much more rarified than the lower, and each Space answering to an Inch of Quicksilver, grows greater and greater; so that the Atmosphere must be extended to a much greater height. Now, upon these Principles, to determine the height of the Mercury at any assigned height in the Air; and è contra, having the height of the Mercury given, to find the height of the Place where the Barometer stands, are Problems not more difficult than curious; and which I thus resolve.