The advantage or power gained by the handle of the pump, is the fame as in the common leaver; that is, as great as the length from the axes of the handle to its end, where the power is applied, exceeds the length of the other part of the handle, from the axes on which it turns, to the pump rod, wherein it is fixed, for lifting the pifton and water.

In the making of pumps, the diameter of the bore, where the bucket works, fhould be proportioned to the height which the pump raises water above the furface of the well, as that a man of ordinary ftrength might work all pumps equally eafy, let their heights be what they will. The annexed table fhews how this may be done, and what quantities of water may be raised in a minute by one man, fuppofing the handle of the pump to be a leaver, increafing the power five times.

Before I quit this fubject, I fhall obferve one thing more to my reader, concerning the preffure of fluids, which is this:

Let a body be ever fo heavy, it may be made to fwim in liquids, by knowing the fpecific gravity.

Thus,

Thus, becaufe the fpecific gravity of gold is to that of water, as 19 to 1; therefore if you hold a guinea to the bottom of a tube of equal diameter (fo as no water can get in) by means of a ftring; then put the tube down in the water, above 19 times the thickness of the guinea in depth, and letting the ftring go, the guinea will not fink, but ride fuftained by the preffure of the fub-adjacent water, which now is ftronger than the force of gravity in the guinea; and thus you make any body fwim, let it be ever fo large and weighty.

SCHOLIUM.

The writers on Hydroftatics demonftrate, that the preffure of liquids on the bottom and fides of veffels, is always proportional to the height thereof, and every way equal at the fame depth.

POABRS

To illuftrate this, let G È. in the annexed figure, be a veffel, from whofe upper part HE proceeds a tall tube, A B C D, communicating therewith. Let this tube and veffel be filled with water, then will the preffure of the water on the bottom G F be as great, and every way the fame, as it would be, were the veffel itfelf as high as the tube, and filledwith water to the level of PS; that is the column of water, A NO B in the prefent cafe has the fame effect on the bottom of the vessel G F, as the column of water P GFS would have.

This is no fmall paradox, but is, notwithstanding that very easy to to conceive; for fince fluids act in every direction, or prefs every way equally; and action and re-action is equal and contrary; it must follow, that the parts of the bottom L N and GL (being equal to N O) will fuftain the fame preffure as N Ò, or as they would do, were the columns 3 G 2

of

of water continued to the height of PQA. For in the line CN, the force of the column of water A O, is exerted on each fide equally, and has the fame effect at IL as at DO, and therefore the lateral preffure being equal, the perpendicular preffures alfo on L N and N O will be equal.

Or thus: It the preffure of the part of I L were lefs than on the part DO, the fluid in the column CO would, by reafon of its greater gravity, have a motion towards the part I L, and the furface A B would defcend: but fince there is a perfect quiefcence of all the parts of the fluid, aud that in the column CO is as much at reft as that in the column C L, it is evident their preffures and effects are every way the fame, and confequently that the column C L preffes as much on the part L N, as the column C O does on the part NO. What is thus proved of the column I N, is to be proved of all the reft, H L, D M, and K F, which makes the propofition manifeft.

This property of fluids is not only in itself very curious, but of great importance in many affairs in life. They who would fee more of this wonderful property, may confult Dr. Grayefande's Elements.

The weight of the whole Atmosphere.

On a fquare inch, it is 15 pounds; on a fquare foot, 2160; on a fquare yard, 19,440; on a fquare mile, 60,217,344,000; and on the whole furface of the earth, and fea together,

12,014,118,565,447,680,000

pounds.

The furface of the body of a middle fized man, is about 14 fquare feet; and as the weight or preffure of the air is equal to 2160 pounds on every fquare foot, on or near the earth's furface, and as the preffure of the air is equal in all manner of directions, its preffure on the whole body of a middle fized man is equal to 30,240 pounds, or 13 tons. because the fpring of the internal air is of equal force with the preffure of the external, the preffure is not felt.

But

The caufe of the ebbing and flowing of the fea, at the fame time, on oppofite fides of the globe.

The reafon why the tides rife on the fide of the earth, which is at any time turned towards the moon, is plain to every one; because her attraction muft occafion à fwelling of the waters towards her on that fide: but the caufe of as great fwell, at the fame time, on the oppofite fide of the earth, which is then turned away from the moon, has been very hard to account for; because the rifing of the tide there is in a direction quite contrary to the attraction of the moon, But this dif ficulty is immediately removed, when we confider, that all bodies mov、 ing in circles, have a centrifugal force, or conftant tendency to fly off from the centers of the circles they defcribe; and this centrifugal force is always in proportion to the diftance of the body from the center of its orbit, and the velocity with which it moves therein,

When

When the body is large, the fide of it which is fartheft from the genter of its orbit will have a greater degree of centrifugal force than the center of the body has; and the fide of it which is nearest the center of its orbit, will have a lefs degree of centrifugal force than its center has.

As the moon goes round the earth every month in her orbit, the earth alfo goes round an orbit every month, which is as much less than the moon's orbit, as the quantity of matter in the moon is lefs than the quantity of matter in the earth, which is 40 times. For, by the laws of nature, when a fmall body moves round a great one, in free and open fpace, both these bodies muft move round the common center of gravity between them,

The moon's mean diftance from the earth's center is 240,000 miles; divide therefore this diftance by 40, the difference between the quantity of matter in the earth and moon, and the quotient will be 6000 miles, which is the distance of the common center of gravity between the earth and moon, from the center of the earth.

Now, as the earth and moon move round the common center of gravity between them, once every month; it is plain, that whilst the moon moves round her orbit, at 240,000 miles from the earth's center, the center of the earth describes a circle of 6000 miles radius, round the center of gravity between the earth and the moon; the moon's attraction balancing the centrifugal force of the earth at its center,

The diameter of the earth is 8000 miles (nearly) and confequently its femi-diameter is 4000: fo that the fide of the earth which is at any time turned towards the moon, is 4000 miles nearer the common center of gravity between the earth and moon, than the earth's center is; and the fide of the earth, which is then fartheft from the moon, is 4000 miles farther from the center of gravity between the earth and moon, than the earth's center is at that time.

Therefore, the radius of the circle deferibed by the parts of the earth which come about towards the moon, by the earth's diurnal motion, is 2000 miles; the radius of the circle defcribed by the earth's center is 6000; and the radius of the circle described by thofe parts of the earth which, in revolving on its axis, are fartheft from the moon, is 10,000

miles.

The centrifugal forces of the different parts of the earth being directly as their distances from the above-mentioned common center of gravity, round which both the earth and moon move, these forces may be expreffed by 2000 for the fide of the earth nearest the moon, by 6000 for the earth's center, and by 10,000 for the fide of the earth which is fartheft from the moon.

But the moon's attraction is greatest on the fide of the earth next her, where the centrifugal force or tendency to fly off from the common center of gravity (and confequently, from the moon) is leaft; and therefore, the tides muft rife on the fide of the earth which is nearest the moon, by the excess of the moon's attraction.

As

As her attraction balances the centrifugal force at the earth's center, it is plain that the centrifugal force of the fide of the earth which is farthest from the moon is greater than her attraction; and therefore, the tides will rife as high upon that fide from the moon, by the excefs of the centrifugal force, as they rife on the fide next her by the excefs of her attraction. And as the earth is in conftant motion on its axis, fò as that any given meridian revolves from the moon to the moon again in 24 hours 50 minutes, each place will come to the two eminences of water, under and oppofite to the moon, in 24 hours 50 minutes, or have two tides of flood and two of ebb in that time. For, as much as the waters rife above the common level of the surface of the fea, under and oppofite to the moon, fo much they must fall below that level half way between the highest places, or at 90 degrees from them.

On thefe principles, it is equally eafy to account for the rifing of the tides, at the fame time, on both fides of the earth; and this rifing is made evident to fight in Mr. Ferguson's Lecture on the Central Forces.

CHRO

CHRONOLOGY.

PART V.

SECTION LXXVIII.

HRONOLOGY is the art of estimating and comparing together the times when any memorable tranfaction hath happened.

It alfo takes a view of the various tracts, calendars, and methods of tomputing time, practifed by different nations; compares them together, and fettles fuch order among them, that the exact time in which any remarkable event happened may be certainly known.

RULES for finding the correfponding years of the Julian Period, with the years of the world, and years before and fince the birth of Chrift; fapposing that the creation of the world was in the 706th year of the Julian Period; and that the birth of Chrift was (according to the vulgar æra thereof) in the 4713th year of the Julian Period.

From any given year of the Julian Period fubtract 706, and the remainder will be the year of the world's age.

If the number of the given year of the Julian Period be less than 4713, fubtract it from 4713; and the remainder will be the number of years before the year of Chrift's birth.

If the given year of the Julian Period is greater than 3967, fubtract 3967 from it; and the remainder will be the number of years after the famous æra of Nabonaffar.

Sub