We are happy to be able to subjoin some ex tremely satisfactory experiments from one of Cap tain Parry's late voyages,} on the comparison of alcohol thermometers by the best London makers, with mercurial ones, which fully confirm our ideas; the differences of the alcohol from the mercury, and with a single exception 57 which perhaps is a misprint for 54) the spirit of wine, are remarkably constant, and the indka tions of the spirit are al ways lower than those of the mercury; the differ ence at -I- 58° being — 5 passion of alcohol = for 180° at higher tem peratures, and though, according to one mode of experiment in Captain Parry's voyage it is made for 180' by observations between + 49.5 and 11.2 — 36.5, yet by the mean of four trials in barometer tubes at temperatures ranging from + 46.5 to — 40 1 it is and what is perhaps most satisfactory, by direct observations on specific gravity between . 1 62 and + 5 it is 9.9 We own that our limits have enabled us to give but a hurried exposition of our opinion on this sub ject, but we hope that enough has been said to show that the subject requires a thorough re-investiga tion.f Before quitting the theory of the thermometer, we must mention an instrument quite original in its idea, and unconnected with the principles that have hitherto been detailed, and therefore requiring a separate place. Dr. Brewster, with that fertility of application in scientific subjects for which he is remarkable, has proposed the measure of the re fractive power of a fluid (which varies with the den sity) as an index of temperature. In this plan, which we believe has not hitherto been published, "a hollow prism is filled with oil or any other fluid which changes its refractive power much with heat. The prism is fixed in a circular scale, and the angular distance between the two stationary points, when the refraction is at a maximum, is a measure of the temperature." The same philoso pher ingeniously proposes the time which a bubble of air takes to pass through a viscid fluid from one end of a cylinder hermetically sealed to the other, as a measure of temperature. Indeed the viscidity of all fluids is sensibly altered by heat; we have seen a species of water clock which is used by the Indians, in which time is measured by the sinking of a copper cup floating in a vessel of water, and having a minute hole in the bottom through which so much water passes in an hour as exactly to make it sink. We have seen experiments made, in which the water employed was at different temperatures, and the time of descent was always diminished as the temperature was raised; this might be in a mi nute degree owing to the increased size of the hole in the copper cup, but principally from the dimin ished viscidity of the fluid.
We must now shortly detail the process of con structing thermometers, and the forms best fitted The tube of which the thermometer is to be blown, is assumed, as will at once be understood from the construction of the instrument, to have a perfectly equable bore, so that equal lengths may indicate equal expansion. This may be proved by causing a column of an inch or two of mercury to pass through the tube, the length occupied by which ought to be the same in every part, and those re jected which are not nearly so; at the same time it were to be desired that instead of aiming at a mere approximation to this accuracy, instrument makers would more frequently proportion the degrees of standard instruments to the absolute capacities of different portions of the tube. As the least mois
ture retards the accuracy of the motion of mercury, and prevents it from occupying the space with nicety, tubes as soon as formed should be sealed at the ends and not opened until they are to be made use of; the operation, too, of drawing the mercury into the tube for measurement, and of blowing the bulb, should by no means be performed with the mouth, but by an elastic caoutchouc bottle fixed to one extremity of the tube, and having a valve in its side. Tubes arc sometimes made with a bore not circular but elliptical, or rather formed like the section of a double convex lens, by which means the height of the mercury becomes easily visible, whilst the area of the bore is no larger than that of a minute circular one, an advantage of no trifling degree, for the difficulty of inspecting the indica tions of a minute capillary thermometer causes an approach of the body, the heat of which affects sur prisingly the accuracy of nice experiments. This we have constantly noticed in observing the sym piesometer, the capillary thermometer of which ought always to be first observed, as the approxi mation of the person of the observer, while making the adjustment of the column of oil, seldom fails, notwithstanding the nicest care, to raise the indi cations of the mercurial bulb, which is nearer the breath of the observer than that of gas, a consider able fraction of a degree.
The proportional adjustment of the diameter of the bulb to that of the bore of the instrument and the proposed size of the degrees, it is a matter of long practice to acquire; it may however be advis able to show the nature of these relations as a mat ter of theoretical importance, and of occasional utility in abstract reasoning on the subject. Some complex formula have, we find, been long since proposed by M. Durand, but we venture to give the following, which we have lately contrived, as be ing, so far as we know, original, and by far the simplest yet suggested; it will be seen that their demonstration is perfectly elementary, yet mathe matically accurate.
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The form of the bulb of thermometers has varied according to their applications. :Most frequently they are spherical, as at Plate DXXIV, Fig. 8., or elongated as Figs. 9 and 10, a form much to be commended as promoting an equilibrium of tem perature through the mercury. So far indeed was this carried in former times, when from the size of the tubes the bulbs were necessarily so very large, that they were made extremely long and coiled up, as shown at Fig. 11, a form which we have often seen adopted both in France and Italy, as in the latter country, flattened bores seem quite unknown. To form the bulb, after the elastic bottle already men tioned has been attached, the other extremity of the tube is heated to fusion and then pressed flat by a piece of metal. The button thus formed is then raised to a white heat and kept constantly turning to prevent it from falling aside; by the action of the bottle the bulb is blown, but is repeatedly re turned to the lamp till the requisite thinness is ob tained, and the desired shape may then be given it, if it is to differ from the spherical; in addition to those already noticed, we may mention the lenticu lar, which is well fitted for exposing a large sur face to the surrounding medium. It was proposed so long ago as by Drebbel, and is still imitated in Italy.