Page:Cyclopaedia, Chambers - Volume 1.djvu/234

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be double, or triple, (go. of the Changes in the common Barometer. This Barometer will fcarce allow its Tube to be inclined to the Horizon at a lefs Angle than 4 5 o, without undergoing the Inconveniency of the Horizontal one.

Wheel-Barometer is a Contrivance of Dr. Hook, to make the Alterations in the Air more fenfible; the Foundation of this is the cotmnonVertical Barometer, with the Addition of a couple of Weights A and B, Fig, ;. hanging in Pu

ingt nd

ot vcrtica pleafure.

on'the Surface of the Mercury in the Tube, and rlfing falling with it. Thus is the Motion of the Mercury communicated, by means of the Pulley, to an Index which turns round a graduated Circle ; and thus the two Inches 1 Afccnt are here improv'd to 4., 5, or more, at But the Friction of the Parts, in the Pulley, and Index, is fo confiderable, that unlefs the Machine be made with a great deal of Accuracy, it does not anfwer.
 * y, the one playing at liberty in the Air, the other r«ft-

'Pendant Barometer is a Machine rather pretty, and curi- ous, than ufeful. It confifts of a conical Tube placed verti- cally, its upper and (mailer Extreme hermetically fealed ; it has no Veflel orCiftern; its conical Figure fupplying that Defect : for when fill'd, like the refl, there will be as much Mercury fuftain'd as is equivalent to the Weight of the At- moff/here ; and as that varies, the fame Mercury takes up a different Part of the Tube, and fo becomes of a different Weight. Thus when the Weight of the Atmofphere is in- creased, the Mercury is driven up into a narrower part of the Tube, by which means its Column is lengthen'd, and, for the Reafon juft given, its Weight increas'd. Again, the Atmrfphere decreafing, the Mercury finks into a wider Part of the Tube, by which means its Column is fhorten'd, and its Preffure accordingly weaken'd. Thus the fame Mer- cury is flill a Balance to the Atmofphere under all its Vari- ations. The Inconvenience in this Barometer is, that to prevent the Mercury and Air from changing Places, the Bore of the Tube muft be very final) ; which Smallnefs of the Bore renders the Friction fo fenfible, as to impede its playing.

Marine Barometer is likewife a Contrivance of Dr. Hook, to be ufed at Sea, where the Motion of the Waves renders the others impracticable. 'Tis nothing more than a double thermometer, or a Couple of Tubes half fill'd with Spirit of Wine; the one hermetically feal'd at both Ends, with a Quantity of common Air inclofed; the other feal'd at one End, and open at the other. Now, the Air, we know, is able to aft on the Spirit of Wine, and to raife it two ways ; the one by its Gravity, as in the 'Torricellian Tube ; the other by its Heat, as in the Thermometer. If then the two Tubes be graduated, fo as to agree with each other at the time when the Air is inclofed, it will eafily follow, that, where-ever the two agree afterwards, the Preffure of the Atmofiihere is the fame, as at the time when the Air was inclofed. If in the Thermometer open to the Air the Liquor Hand higher, confidering withal how much the other is rifen or fallen from the other Caufe of Heat or Cold, the Air is heavier ; on the contrary, when it is lower, compar'd with the other, the Air is lighter than at the time when the Inftrument was graduated. Here the Spaces an- fwering to an Inch of Mercury will be more or lefs, accor- ding to the Quantity of the Air inclofed, and the Smallnefs of the Tubes ; and may be increas'd, almofl in any Pro- port'on.

But it muft be remember'd, that the Denfity and Rari- ty of the Air, on which this Machine is founded, don't on- ly depend on the Weight of the Atmofphere, but alfo on the Action of Heat and Cold. This, therefore, can never be a juft Barometer ; but may properly enough be call'd a Menofcope, or Inftrument to {new the Denfity of the Air.

Statical Barometer or Barofcope, ufed by Mr. Boyle, Ot- to, Gucrric, &c. is fallacious, and liable to be acted on by a double Caufe : It confifts of a large Glafs Bubble, ba- lanced by a Brafs Weight, in a nice Pair of Scales : for thefe two Bodies being of equal Gravity, but unequal Bulk, if the Medium in which they equiponderate be changed, there will follow a Change of their Weight ; fo that if the Air grows heavier, the greater Body being lighter in Spe- cie, will lofe more of its Weight than the leffer and more compact; but if the Medium grow lighter, then the big- ger Body will outweigh the lefs.

But the moft accurate Barometer ever yet invented,is that of Mr.CVr/Tff // ; the Structure whereof he defcribes as follows: Suppofe ABCD, Fig. 6. a Bucket of Water, wherein is the Barometer x r e z y s m, confiding of a Body xr s m, and a Tube c zy 0. The Body and Tube are both concave Cylinders, communicating with each other, made of Tin, or rather Glafs. The Bottom of the Tube, zy, has a Lead Weight to fink it, fo as the Top of the Body may juft fwim

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guilb it from the other at Bottom, which we call the Tube : this Pipe is to fuftain the Inftrument from finking to the Bottom, md is a Wire, ms, tie two Threads oblique to the Surface of the Water, performing the Office of Diagonals. Now, while the Inftrumenr finks more or lefs, by the Alte- ration of the Gravity of the Air ; there, where the Surface of the Water cuts the Thread, is form'd a fmall Bubble, which afcends up the Thread, as the Mercury of the com- mon Barometer afcends, (g vice verfa. This Instrument, as appears from a Calculation which he annexes, /hews the Al- terations in the Air more accurately than the common Baro- meter, by 1200 times. He obferves, that the Bubble is fel- dom known to {land flill a Minute ; that a fmall Blaft of Wind, that can't be heard in a Chamber, will make it fink fenfibly ; that a Cloud always makes it defcend, igc. c Phi£nomcna of the Barometer.

The Phenomena of the Barometer are various ; the Caufes aflign'd for 'em by feveral Authors as various ; nor is its Ufe in predicting the Weather yet perfectly afcer- tain'd.

On the top of Snowdon Hill, 1240 Yards high, Dr. Hal- ley found the Mercury lower by 3 Inches 8 Tenths than at the Foot thereof; whence it appears, that at every ;o Yards the Mercury finks T ' v of an Inch. Mr. Derham, from fome Experiments he made at the Top and Bottom of the Monument, allows 3 2 Feet perpendicular Afcent to a Fall of the Mercury of J ^ of an Inch: Whence we have not only a Foundation for determining the Height of the Atmofphere, which on this Foundation (were it equally denfe every- where) would not be found more than 5 Miles and ;-; bur alfo a very accurate Method of meafuring the Height of Mountains. Thus, if on the Surface of the Earlh the Mercu- ry be at 30 Inches, at 1000 foot high, it will be at 18,91 Inches; at 2000 Foot, 27,8s ; at 3000, 25,85 ; at 4000, 25,87; at 5000, 24,93 ; at 1 Mile, 24,47; at 2 Miles, 20,29 5 at 5 Miles, 1 1,28 ; at 10 Miles, 4,24 ; at 1 5 Miles, 1,60; at 20, 0,95 ; at 30 Miles, 0,08 ; at 40, 0,012 : tho it muft be obferv'd, this is on a Supposition that the Atmo- fphere is equally denfe every where.

The greateft Height the Mercury has been known to

{land at in the Barometer, at London, is 30 Inches |, its leaft 28 Inches ; its greateft Height at the Obfervatory at 'Paris, has been found 28 Inches £-., and its leaft 16 fj of the 'Paris Foot, which exceeds the London Foot by -^ : and with thefe Obfervations agree others made at Halt, in Saxony, by Wolfius. 'Tis true, there is an Experiment wherein the Height of the Mercury is found fuprizingly to exceed thefe Numbers ; Mercury perfectly purged being fufpended in a Tube, in the-Torricellian way, at the Height of 7 5 Feet ; tho by the leaft Shake it falls down to the or- dinary Height. See the Accounts of this Phenomenon un- der the Word Torricellian.

The Phenomena of the Barometer, Mr. Boyle obferves, are fo very precarious, that 'tis exceedingly difficult to form any general Rules about the Rife or Fall thereof. Even that which feems to hold moft univerfally, viz. That when high Winds blow the Mercury is the lower, fometirnes fails.

Z)jr. Halley gives us the following Obfervations.

That in calm Weather, when the Air is inclined to Rain, the Mercury is commonly low ; in ferene good fettled Wea- ther, high.

That* on great Winds, tho unaccompanied with Rain, the Mercury is the loweft of all, with regard to the Point of the Compafs the Wind blows on. That, ceteris paribus, the greateft Heights of the Mercury are on Eaflerly and North-Eaflerly Winds. That after great Storms of Wind, when the Mercury has been low, it rifes again very {aft.

That in calm frofty Weather ir {lands high.

That the more Northerly Places find greater Alterations than the more Southern : And that within the Tropicks, and near them, there is little or no Variation of the Height of the Mercury at all.

Dr. Beal obferves ;

That, C£teris paribus, the Mercury is higher in cold Weather than in warm, and ufually in the Morning and Evening higher that at Mid-Day.

That in fettled and fair Weather, the Mercury is higher than either a little before or after, or in the Rain ; and that it generally defcends lower after Rain than it was be- fore it. If it chance to rife higher after Rain, is is gene- rally follow'd by a fettled Serenity.

That there are frequently great Changes in the Air, with- out any perceptible Alteration in the Barometer.

For the Ufe of Barometers an ingenious Author obferves,

That by their means we may regain the Knowledge which

flill refides in Brutes, and which we forfeited bynotconti-

with the Surface of the* Water, by the addition of nuing in the open Air, as they generally do ; and by our

fome Grain Weights a-top. The Water, when the In- ftrument is forced with its Mouth downwards, gets up into the Tube to the Height y 0. There is added on the Top a fmall concave Cylinder, which we call the 'Pipe, to diftin-

Intemperance, corrupting the Crafis of our Senfes.

As to the Predictions from Barometers, Dr. Halley has found, that the Rifing of the Mercury forebodes fair Wea- ther after foul, and an Eaflerly or North-Eaflerly Wind.

That