But now we have had occasion to mention the difference there is between the height of the _Mercury_ at one time, from the height thereof at another, it may not be unacceptable to offer at some Reasons for the said difference; which, at least to my self, seem to have some appearance of Truth. _First_, Then it's undoubtedly demonstrable, that the height of the Cylinder of _Mercury_ is equal to the weight of the whole incumbent Air, and consequently that that whole is sometimes a fifteenth more than at other times; which cannot otherwise be, but by the access of new Matter when 'tis heavy, and its diminution when 'tis light; that _Hypothesis_ therefore that shews how the Air shall be encreased or diminished, in any particular place, will give a Reason for the greater and lesser height of the _Mercury_ in the _Baroscope_: But to direct us in the choice of the several Causes, which may be assign'd for the Increase and Decrease of the Air, 'twill not be unnecessary to enumerate some of the principal Observations made upon the _Barometer_, most whereof are sufficiently known already to all those that are curious in these Matters. The _First_ is, That in calm Weather, when the Air is inclin'd to Rain, the _Mercury_ is commonly low. 2. That in serene good settled Weather, the _Mercury_ is generally high. 3. That upon very great Winds, though they be not accompanied with Rain, the _Mercury_ sinks lowest of all, with relation to the Point of the Compass the Wind blows upon. 4. That _cæteris paribus_ the greatest heights of the _Mercury_ are found upon Easterly and North-Easterly Winds. 5. That in calm frosty Weather the _Mercury_ generally stands high. 6. That after very great Storms of Wind, when the _Quicksilver_ has been low, it generally rises again very fast. 7. That the more Northerly places have greater Alterations of the _Baroscope_, than the more Southerly. 8. That within the _Tropicks_ and near them, those Accounts I have had from others, and my own Observation at St. _Helena_, make very little or no Variation of the height of the _Mercury_ in all Weathers. Now that Theory that can well account for all these appearances, will, in all probability, approach nearer the true cause of the _Barometers_ Variations, than any thing hitherto afforded; and such an one I am bound to believe, is that which I here lay down with submission to better Judgments. I conceive that the principal Cause of the rise and fall of the _Mercury_, is from the variable Winds, which are found in the _Temperate Zones_, and whose great unconstancy here in _England_ is most notorious. I shall not at present inquire into the Cause of its uncertainty, but the Matter of Fact being most undoubted, the Legitimate Consequences thereof must be allow'd me, let it proceed from what it will.