This was evidently an improvement on the air-thermometer, but was itself not free from objections. The liquor could not easily be obtained of the same strength, and hence different tubes filled with it, when exposed to the same degree of heat, would not correspond. Another defect was, the want of sonic fixed guide as a standard to commence the graduation. Philosophers soon saw that some fixed and unalterable point must be found, by which all thermometers might.,be accurately adjutsed. Dr. Halley propos ed that thermometers should be gra duated in a deep pit, where the tempera ture in all seasons was nearly the same. This however could not generally be prac tised. Ile thought of the boiling point of water, of mercury, and of spirit of wine, preferring the latter, on account of the freezing of water, not knowing that this was fixed and uniform. At length Sir Isaac Newton determined this important point, on which the accura cy and value of the thermometer depends. lie chose, as fixed, those points at which water freezes and boils ; the very points which the experiments of succeeding philosophers have determined to be the most fixed and convenient. Sensible of the disadvantages of spirit of wine, he tried another liquor which was homogeneous enough, and capable of a considerable rarefaction, several times greater than spirit of wine. This was linseed oil. It has not been observed to frieze even in very great colds, and it bears a heat very much greater than water before it boils. With these advantages it was made- use of by Sir Isaac Newton, who discovered by it the comparative degree of heat for boiling water, melting wax, boiling spirit of wine, and melting tin ; beyond which it does not appear that this thermometer was applied. The method he used for adjusting the scale of this oil-thermome ter was as follows 3 supposing the bulb, when immerged in thawmg-snow, to con tain 10,000 parts, he found the oil expand by the heat of the human body, so as tit take up one thirty-ninth more space, or 10,256 such parts ; and by the heat of' water boiling strongly 1t;,725; and by the heat of melting tin 11,516. So that reckoning the freezing point as a common limit between heat and cold, he began his scale there, marking it 0, and the heat of the human body he made ; and consequently, the degress of heat be ing proportional to the degrees of rare faction, or 256 :725:: 12 : 34, this num ber 34 will express the heat of' boiling wa ter; and by the same rule, 72 that of melt ing tin. This thermometer was con structed in 1701. To the application of oil as a measure of heat and cold, there are insuperable objections. It is so vis cid, that it adheres too strongly to the sides of the tube. On this account it as cends and descends too slowly in case of a sudden heat or cold. In a sudden cold, so great a portion remains adhering to the sides of the tube after the rest has subsi ded, that the surface appears lower than the corresponding temperature of the air requires. An oil thermometer is there fore not a proper measure of heat and cold. All the thermometers hitherto proposed were liable to many inconveni ences, and could not be considered as ex act standards fin- pointing out the various degrees of temperature. This led Reau mur to attempt a new one, an account of which was published in the year 1730 in the Memoirs of the Academy of Sciences. This thermometer was made with spirit of wine. He took a large ball and tube, the dimensions and capacities of which were known ; he then graduated the tube, so that the space from one division to ano ther might contain 1,000th part of the li quor; the liquor containing 1,000 parts when it stood at the freezing point. He adjusted the thermometer to the freez ing point by an artificial of water ; then putting the ball of his ther mometer and part of the tube into boiling water, he observed whether it rose 80 di visions ; if it exceeded these, he changed his liquor, and by adding water lowered it, till upon trial it should just rise 80 di visions ; or if the liqour, being too low, fell short of eighty divisions, he raised it by adding- rectified spirit to it. The li quor thus prepared suited his purpose, and served for making a thermometer of any size, whose scale would agree with his standard. At length a different fluid was proposed, by which thermometers could be made free from most of the de fects hitherto mentioned. This fluid was mercury, and seems first to have oc curred to Dr. Halley, but was not adopted
by him, on account of its having a smaller degree of expansibility than the other fluids used at that time.
The honour of this invention is general ly given to Fahrenheit of Amsterdam, who presented an account of it to the Royal Society of London in 1724. That we may judge the more accurately of the propriety of employing mercury, we will compare its qualities with those of the fluids already mentioned, air, alcohol, and oil. Air is the most expansible fluid, but it does not receive nor part with its heat so quickly as mercury. Alcohol does not expand much by heat. In its ordinary state it does not bear a much greater heat than 175° of Fahrenheit ; but when highly rectified, it can bear a greater degree of cold than any other liquor hitherto employed as a measure of temperature. At Hudson's Bay, Mr Mac nab, by a Mixture of vitriolic acid and snow, made it to descend to 69° below 0 of Fahrenheit. There is an inconvenience, however, attending the use of this liquor; it is not possible to get it always of the same degree of strength. As to oil, its expansion is about 15 times greater than that of alcohol ; it sustains a heat of 600% and its freezing point is so low that it has not been determined ; but its viscosity renders it useless.
Mercury is far superior to alcohol and oil, and is much more manageable than air. 1. As far as the experiments al ready made can determine, it is, of all the fluids hitherto employed in the construc tion of thermometers, that which mea sures most exactly equal differences of beat by equal differences of its bulk its dilatations are, in fact, very nearly pro portional to the augmentations of heat applied to it. 2. Of all liquids it is the most easily freed from air. 3. It is fitted to measure high degrees of heat and cold. It sustains a heat of 600° of Fahrenheit's scale, and does not congeal till it fall 39 or 4.0 degrees below 0. 4. It is the most sensible of any fluid to heat and cold, even air not excepted. Count Rumford finmd, that mercury was heated from the freezing to the boiling point in 58 seconds, while water took 2 minutes 13 seconds, and common air 10 minutes and 17 se conds. 5. Mercury is a homogeneous fluid, and every portion of it is equally dilated or contracted by equal variations of heat. Any one thermometer, made of pure mercury, is, exteris paribus, pos sessed of the same properties witlLevery other thermometer made of pure mer cury. Its power of expansion is indeed about six times less than that of spirit of wine, but it is great enough to answer most of the purposes fin- which a ther mometer is wanted. The fixed points, which are now universally chosen for ad justing thermometers to a scale, and to one another, are the boiling and freezing water points. The boiling water point, it is well known, is not an invariable point, but varies some degrees, according to the weight and temperature of the atmosphere. In an exhausted receiver, water will boil with a heat of 98° or 100°; whereas, in Papin's digester, it will ac quire a heat of 412°. Hence it appears, that water will boil at a lower point, ac cording to its height in the atmosphere, or to the weight of the column of air' which presses upon it. In order to en sure uniformity, therefore, in the con struction of thermOmeters, it is now agreed, that the bulb of the tube be plunged in the water when it boils vio lently, the barometer standing at 30 En glish inches, and the temperature of the atmosphere 55°. A thermometer made in this way, with its boiling point at 212°, is called, by Dr. Horsley, " Bird's Falwell belt," because Mr. Bird was the first person who attended to the state of the barometer in constructing thermometers.
As artists may be often obliged to ad just thermometers under very different pressures of the atmosphere, philoso phers have been at pains to discover a general rule, which might be applied on all occasions. M. de Luc, from a series of experiments, has given an equation for the allowance on account of this differ ence, in Paris measure, which has been verified by Sir George Schuckburg ; also Dr. HorSiey, Dr. Maskelyne, and Sir George Schuckburg, have adapted the equation and rules to English measures, and have reduced the allowances into ta bles, for the use of the artist. Dr. Hors ley's rule, deduced from De Luc's, is this: 99 --- log. s-92.804 h.