equal Parts, which is as far as the moft fcrupulous Philofopher can defire.

Euphrof. All this may be true enough for ought I know; but unless you can explain it to me by an Example, you may as well tell the Story of your Vernier to my Linnet, as to me.

Cleon. I make no doubt but you will eafily understand it by an Example or two, which you fhall perform yourself, that you may be the more ready at it another Time.

Euphrof. That pleafes me well:-But, what am I to do firft?

Cleon. Caft your Eye on the Barometer, and move the Index of the Vernier nicely to the Edge of the Quick-filver in the Tube.

Euphrof. This I have done.

Thing to be obferved?

What is the next

Cleon. Look the Divifion on the Scale, that is next below the Index of the Vernier, and tell me what it is. Euphrof. I will look.- -It is the fecond Divifion above 30; by which, I perceive, the Quick-filver is more than 30 Inches and of an Inch high.

Cleon. Very right; and now you are to tell me how much more, by obferving what Divifions of the Vernier and Scale coincide, which you are next to fee.

Euphrof. I obferve, the fourth Divifion upon the Vernier is juft against the Divifion upon the Scale.But what do I learn from thence?

Cleon. From thence it is apparent, that the Index is advanced four Parts out of Ten, from the second to the third Divifion of the Scale above 30.- -And there

fore, the whole put together amounts to this, that the Quick-filver in the Tube ftands at the Height of 30 Inches of an Inch, and 4 of another; that is, 30 Inches, and 24 hundredth Paits of another Inch, is the Height of the Quick-filver.

Euphrof. Very good; I fee by this plainly, that I can determine the Height or Motion of the Quick-filver to the Hundredth Part of an Inch; which I am fure will be fufficient for my Purpofe; and, I fuppofe, for any one's elfe. Is there any Thing farther, in regard to Barometers, worth my Notice at this Time?

Cleon. I could obferve to you many other Circumftances, and Particulars, relative to Barometers, but I judge it would be Lofs of Time, at prefent, as my only View in thefe Conferences with you, is to make you acquainted with that which is beft, and most perfect in its Kind, among the Inftruments used in every Art or Science. We fhall therefore difmifs this Subject, and proceed to confider another Inftrument, by which we explore the various Degrees of Heat and Cold in the Air, for the Subject of the next Leifure-hour which

occurs.

DIALOGUE IX.

Of the Nature and Ufe of THERMOMETERS, and their Variety of different Conftructions.

I

Euphrofyne.

Have now an Opportunity, Cleonicus, of putting you in Mind of your Promife of explaining to me the Nature of the THERMOMETER, by which you measure the Degrees of Heat and Cold in the Air.

Clean. No-body is more ready to fulfil Engagements of that Kind, than myfelf, and I fhall embrace every Opportunity for that Purpofe; especially the prefent one. The Nature of the Thermometer is, undoubtedly, reckoned among the pleafanteft Speculations in Philofophy. As the Defign of this Inftrument is to give us an Eftimate of the Heat or Cold in the Air at all Times, it is evident, it must be of the utmoft Confequence upon many Accounts; fince the Health, and confequently the Life, of Man is greatly affected by this Quality of the Air; I need not give you Inftances of the manifold Miferies, Difeafes, and other Misfortunes that have happened to Mankind in every Age, from the Intemperature, and Extremity of Heat and Cold in the Air, and therefore, we cannot be too fufficiently instructed in all the proper Methods of guarding against them; and not only fo, but the Vegetation of different Kinds of

3

Plants

Plants and Trees depends upon a peculiar Degree of Warmth or Heat in the Air, Water, or Earth, in which they grow, and the Thermometer is found to be the principal Inftrument for all fuch Purposes.

Euphrof. As this is the Cafe, it will undoubtedly follow, that the Ingenuity of Men of a philofophic Turn, muft have contriv'd and introduced many different Forms and Structures of this Inftrument; I fhould be glad, therefore, to know what they are, and how many?

Cleon. I muft obferve to you, in the first Place, the Rationale of the Inftrument in general, and then recount to you some of the beft Methods of conftructing a Scale for its Ufe.In the firft Place, therefore, you are to observe, that every Caufe is beft eftimated or measured by the Effects which it produces; and fince, in most Bodies, the Effect of Heat is an Increase of their Dimenfions, therefore the variable Bulks, or Dimenfions of Bodies, will be a proper Measure for this Purpose, and thofe, whofe Dimenfions are moft of all increased, or diminished, by Heat or Cold, first offer themfelves as the propereft Subjects to answer this End; and fince Fluids are much more fubject to a Dilatation and Contraction of their Bulk, than folid Bodies, our first and laft Effays of this Kind have been made with them.

Euphrof. Among the different Kinds of Fluids, which have been moftly used for the Measures of Heat and Cold?

Cleon. The AIR is a Fluid, moft of all apt to rarify with Heat, and be condenfed with Cold, and would ferve for the Medium of a Thermometer the beft of any Thing in Nature, were it not that the fame Effects are produced from other different Causes. According to the different Gravity of the Air it will occupy a greater or leffer Space, as well as from different Degrees of Heat and Cold, and fo the Experiment of a Thermometer with Air, would become ambiguous, as it would be oftentimes doubtful, whether the Effect was owing to one, or the other Caufe.

Euphrof. And are there any Sort of Fluid Bodies which will be extended or contracted by Heat or Cold alone?

Cleon. Yes; every other Fluid but Air, as all others are deftitute of a Spring, or Elafticity, and are incompreffible from any Weight on their Surfaces, the Alteration, therefore, that they undergo in their Bulks must be from Heat and Cold alone; among thefe, Water expands and contracts but little, and therefore not fo fit a Subject for a Thermometer; befides, it has been lately found, than an Increase in the Bulk of Water will be occafioned by a certain Degree of Cold, as well as generally by Heat, which render it ftill more unfit for this Purpose. But that which most of all forbids the Ufe of Water in a Thermometer is, that it will not retain its Fluidity fo long as is neceffary, but freezes, congeals, or becomes fixed by a certain Degree of Cold, and then breaks the Inftrument, by its increas'd Bulk.

Euphrof. There are different Sorts of Oils, which, I prefume, may have all' ftood a Scrutiny in this Refpect; pray, what has been the Succefs in regard to them?

Cleon. Our Virtuoft have fucceeded greatly in their Thermometers, constructed with Oil, particularly LinseedOil, which as it requires a much greater Heat to make it boil than Water does, or Spirits, or that which melts Wax, Tin, and Lead; but as it can be applied to no greater Degrees of Heat than thofe, and moreover, as the Oil, by its vifcid Quality, is apt to stick to, and foul the Tube in which it is contained, the Oil Thermometer is of late but very little in Ufe, though it has been rendered very famous, as it has been made a Standard for Experiments on the Weather, by the great Sir Isaac Newton; and they are ftill of confiderable Ufe, where great Degrees of Heat are not required, as in HotHoufes, &c.

Euphrof. But is not that Fluid, which we usually call a Spirit, a proper Subject for thefe Purposes? Do not Spirits of Wine, for Inftance, expand and contract with Heat and Cold, and thereby anfwer the Virtuofo's Purpose, in a very neat and elegant Manner?

Cleon. It will do fo, and accordingly has been, and ftill is ufed for that Purpofe; but fome Objections lie against the Ufe of Spirit; as firft, it being colourless, it is neceffary to tinge it with Cochineal, or fome fuch

Matter,