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So that the weight of the quicksilver in the tube, above that in the bason, is at all times equal to
737 the weight or pressure of the column of atmosphere above it, and of the same base with the tube ; and hence the weight of it may at all times be computed; being
30 nearly at the rate of half a pound avoirdupois for every inch of quicksilver in the tube, on every square inch of base ; or exactly it is of a pound on
29 the square inch, for every inch in the altitudeof the quicksilverweighs just alb, or nearly a pound, in the mean temperature of 55° of
-28 heat. And consequently, when the barometer stands at 30 inches, or 24 feet high, which is nearly the medium or standard height, the whole pressure of the atmosphere
127 is equal to 14 pounds, on every square inch of the base ; and so in proportion for other heights.
OF THE THERMOMETER.
390. THE THERMOMETER is an instrument for measuring the temperature of the air, as to heat and cold.
It is found by experience, that all bodies expand by heat, and contract by cold ; and hence the degrees of expansion become the measure the degrees of heat. Fluids are more convenient for this purpose than solids : and quicksilver is now most commonly used for it. A very fine glass tube, having a pretty large hollow ball at the bottom, is filled about half way up with quicksilver : the whole being then heated very hot till the quicksilver rise quite to the top, the top is then hermetically sealed, so as perfectly to exclude all communication with the outward air. Then, in cooling, the quicksilver contracts, and consequently its surface descends in the tube, till it come to a certain point, correspondent to the temperature or heat of the air. 'And when the weather becomes warmer, the quicksilver expands, VOL. II. нь
and its surface rises in the tube ; and
100 tube ; observing at what division of the scale the top of the quicksilver stands. And the method of preparing the scale, as used in England, is thus :- Bring the thermometer into the temperature of
39freezing, by immersing the ball in water just freezing, or in ice just thawing, and mark the scale where the mercury then
20 stands, for the point of freezing. Next, immerge it in boiling water; and the quicksilver will rise to a certain height 10in the tube ; which mark also on the scale for the boiling point, or the heat of boiling water. Then the distance between these two points, is divided into 180 equal divisions, or degrees ; and the like equal degrees are also continued to any extent below the freezing point, and above the boiling point. The divisions are then numbered as follows ; namely, at the freezing point is set the number 32, and consequently 212 at the boiling point; and all the other numbers in their order.
This division of the scale is commonly called Fahrenheit's. According to this division, 55 is at the mean temperature of the air in ihis country ; and it is in this temperature, and in an atmosphere which sustains a column of 30 inches of quicksilver in the barometer, that all measures and specific gravities are taken, unless when otherwise mentioned; and in this temperature and pressure the relative weights, or specific gravities of air, water, and quicksilver, are as
tor air, and these also are the weights of a cu. 1000 for water,
bic foot of each, in avoirdupois ounces, 13600 for mercury; ( in that state of the barometer and thermometer. For other states of the thermometer, each of these bodies expands or contracts according to the following rate, with each degree of heat, viz.
Air about - Aft part of its bulk,
ON THE MEASUREMENT OF ALTITUDES BY THE
BAROMETER AND THERMOMETER.
391. FROM the principles laid down in the scholium to prop 76, concerning the measuring of altitudes by the barometer, and the forgoing descriptions of the barometer and thermometer, we may now collect together the precepts for the practice of such measurements, which are as follow :
First. Observe the height of the barometer at the bottom of any height, or depth, intended to be measured ; with the temperature of the quicksilver, by means of a thermometer attached to the barometer, and also the temperature of the air in the shade by a detached thermometer.
Secondly. Let the same thing be done also at the top of the said height or depth, and at the same time, or as near the same time as may be. And let those altitudes of barometer be reduced to the same temperature, if it be thought necessary, by correcting either the one or the other, that is, aug. ment the height of the mercury in the colder temperature, or diminish that in the warmer, by its goöopart for every degree of difference of the two.
Thirdly Take the difference of the common logarithms of the two heights of the barometer, corrected as above if necessary, cutting off 3 îngures next the right hand for decimals, when the log-tables go to 7 figures, or cut off only 2 figures when the tables go to 6 places, and so on; or in general remove the decimal point 4 places more towards the right hand, those on the left hand being fathoms in whole numbers.
Pourthly. Correct the number last found for the difference of temperature of the air, as follows; Take half the sum of the two temperatures, for the mean one : and for every degree which this differs from the temperature 31°, take so many times the as part of the fathoms above found, and add them if the mean temperature be above 31°, but subtract them if the mean temperature be below 31°; and the sum or difference will be the true altitude in fathoms : or, being multiplied by 6, it will be the altitude in feet.
392. Erample 1, Let the state of the barometers and thermometers be as follows; to find the altitude, viz. Barom. Thermom.
attach. detach. Ans. the alt. is
393. Exam. 2. To find the altitude, when the state of the barometers and thermometers is as follows, viz. Barom. Thermom.
attach| detach. Ans the alt. is Lower 29 45 38 31
409, fathoms. Upper 26.82 41 35
or 2458 feet,
ON THE RESISTANCE OF FLUIDS, WITH THEIR
FORCES AND ACTIONS ON BODIES
394. If any Body Move through a Fluid at Rest, or the Fluia
Move against the Body at Rest; the Force or Resistance of the Fluid against the Body, will be as the Square of the Pelocity and the Density of the Fluid. That is, R a dv.
For the force or resistance is as the quantity of matter or particies struck, and the velocity with which they are struck. But the quantity or number of particles struck is any time, are as the velocity and the density of the fluid. Therefore the resistance, or force of the fluid, is as the density and square of ihe velocity.
395. Corol. 1. The resistance to any plane, is also more or less, as the plane is greater or less; and therefore the resistance on any plane, is as the area of the plane a, the density of the medium, and the square of the velocity. That, is Raadva.
396. Corol. 2. If the motion be not perpendicular, but oblique to the plane, or to the face of the body, then the resistance, in the direction of motion, will be diminished in the triplicate ratio of radius to the sine of the angle of inclination of the plane to the direction of the motion, or as the cube of radius to the cube of the sine of that angle. So that Roc adv233, putting 1 radius, and : a sine of the angle of inclination CAB.
For, if all be the plane, ac the direction of motion, and Bc perpen
c. dicular toac; then no more particles meet the plane than what meet the perpendicular Bc.and therefore their number is diminished as AB 10 BC or
B as I to s. But the force of each par
ticle, striking the plane obliquely in the direction ca, is also diminished as AB to BC, or as I to s; therefore the resistance, which is perpendicular to the face of the pline by art. 52, is as 1 to 82. But again, this resistance in the direction perpendicular to the face of the plane, is so that in the direction Ac, by art. 51, as AB to BC, or as 1 10 8. Consequently, on all these accounts, the resistance to the plane when moving perpendicular to its face, is to that when moving obliquely, as 13 to 83, or I 10 83.
That is, the resistance in the direction of the motion, is diminished as 1 to 89, or in the triplicate ratio of radius to the sine of inclination.
399. The Real Resistance to a Plane, by a Fluid acting in a
Direction perpendicular to its Face, is equal to the Weight of a Column of the Fluid, whose Base is the Plane, and Altitude equal to that which is due to the Velocity of the Motion, or through which a Heavy Body must fall to acquire that Velocity.
The resistance to the plane moving through a fluid, is the same as the force of the fluid in motion with the same velocity, on the plane at rest But the force of the fluid in motion, is equal to the weight or pressure which generates that motion ; and this is equal to the weight or pressure of a column of the fluid, whose base is the area of the plane, and its altitude that which is due to the velocity.
398. Corol. 1. If a denote the area of the plane, v the velocity, n the density or specific gravity of the fluid, and g= 1671 feet, or 193 inches. Then the altitude due to the velocity v being therefore a XnX
4g be the whole resistance, or motive force r.
399. Corol. 2. If the direction of motion be not perpendicular to the face of the plane, but oblique to it, in any angle, whose sine is 8. Then the resistance to the plane will be
4g 400. Corol. 3. Also, if w denote the weight of the body, whose plane face a is resisted by the absolute force R ; then
and283 the retarding force f, or will be
401. Corol. 4. And if the body be a cylinder, whose face