We shall now describe as concisely as possible the principal modifications of the simple thermom eter ; the most interesting of these are termed self registering, which by inspection denote the greatest heat or cold which has occurred since the last ob servation, or else are employed to note the tempera ture at any moment in the absence of the observer. Of the former kind, the first which deserves notice, though Bernouilli seems to have the merit of orig inating the idea, is the thermometer of Lord Charles Cavendish.t For the maximum, he employed a mercurial thermometer, A b, Fig. 13, in which the temperature at the time of observation was indi cated by the height of the mercury at C, while resting upon it was a column of alcohol which filled the tube. The termination of the bore was capillary, and covered by a glass receptable b, into which the capillary termination projected about two-thirds of the depth of the cap. By a rise of temperature, the mercury forces a portion of the spirits over the extremity of the tube, and as it cools, sinks without the possibility of the column of spirit carrying back what was expelled; the empty space of the tube, therefore, indicates the excess of the highest temperature since the last observation, above that indicated by the actual height of the mercury. It is adjusted by reversing the instrument and heating the bulb with the hand till the column unites with the spirit in the cap b, when the termination of the tube being covered, in this position it is allowed to cool, and the column is again made continuous. There arc two defects in this instrument, the time required to assume the temperature of the air in the adjustment, which is obviously necessary, and that when in a reversed position and contracting, the mercury allows some of the spirit to pass it. We have found it useful in employing this thermometer to cool the bulb by means of ether, more speedily than it would other wise have done to the temperature of the air. The minimum thermometer of the same philosopher was far more complex, and we suspect nearly im practicable. Its principle is represented at Plate DXXIV. Fig. 14, where the bulb A is filled with spirit, and also the bulb B, except a small portion of the mercury in the bottom, a column of mercury commences in the leg of the syphon a b, and indi cates the temperature by its height at C. As the temperature declines, the mercury at b is drawn up into the bulb B into which it is intended to trickle by minute drops, the neck of communica tion a having a fine glass thread within it; a plan ingenious in theory, but we fear almost unattaina ble for practical purposes of adjustment, more especially as when it was to be prepared for a new observation, it was expected that on being inclined, the mercury would trickle back through a, and pass ing the spirits which had occupied it, reunite with its own column, a supposition rather startling to those who are acquainted with the motions of mer cury and spirit.
A modification of Lord Charles Cavendish's ther mometers, combining both a maximum and mini mum register, in which the contrivance for the latter is greatly simplified, has recently been pro posed by Mr. Forbes,* and a figure of it is given at Fig. 15, where a is the bulb filled with spirits, and having a little mercury in the bottom, being of the peculiar form there represented. The tube which terminates with it in a capillary orifice c is filled with mercury as far as h, which marks upon a scale the true temperature of the moment: above the mercury is a column of spirit, which by means of a capillary orifice, and cover d, indicates the maxi mum as in the thermometer just described. The portion of the tube ef, which is filled with spirit at any observation, of course indicates the quantity of mercury expelled into the bulb, and consequently by the application of a scale, the greatest depres sion of temperature. The instrument is adjusted by reversing it, when the spirit will cover the tip per orifice, and the mercury the lower one; the hand is applied to the bulb till the spirit column is joined, and ether is then poured upon it till the mercurial column is brought in contact with the magazine in the bulb, the hand may be again em ployed to raise the temperature to that of the air as previously observed by the height h. By this means the instrument will be much quicker ad justed than even in Lord C. Cavendish's maximum, for which also we have found it advisable to use ether to reduce it quickly by the actual tempera ture. Notwithstanding the immense superiority in the simplicity of this instrument to the mini mum one above described, we suspect from some attempts we have made on the subject, that there arc almost insupportable practical obstacles to the satisfactory or general use of this thermometer, however accurate it may be in theory.
The thermometer which succeeded those of Cavendish was that of Mr. Six, described in the Philosophical Transactions for 1782.f It is repre sented in Fig. 16, where a represents an elongated bulb filled with spirit, which is continued through the curve b down to h, where mercury begins, and occupying the lower bend of the tube, reaches up to i, where the spirit recommences, and partly fills the expansion of the tube c, so as to give it space for its enlarged bulk when the temperature of the bulb a is raised. Placed within the spirit above its
contact with the mercury are the two indices e, f, which are composed of steel coated with glass, and terminated with a dot of enamel on each end; to prevent them moving through the spirit by their own weight, a spring of glass was attached to them, for which now a bristle is more generally employed. It is obvious that, by the motion of the mercurial column as the spirit in the bulb con tracts and expands, the indices will be left in the tube at different heights e and f, the indications of which are ascertained by two scales shown in the figure, the one of which is of course numbered downwards, the other upwards. The adjustment is made by means of a magnet, which brings down the indices into contact with the mercurial column.
This instrument, will be observed, is not a little complex in its parts, having two contacts of spirit and mercury, and two indices, all which are detrimental to the practical accuracy of the ther mometer. On this account the instrument has usually been made on a large scale, to prevent de rangement in working; Mr. Six made the bulb no less than sixteen inches long, and half an inch diame ter, while the tube had a bore of -2 inch. But this enormously clumsy instrument had from its size still greater defects for any delicate purposes, and has since been much reduced; an instrument of this description, which we used almost constantly for nearly two years, made by one of the best London makers, had a bulb of only six inches in length. Yet we cannot help noticing the extremely slovenly manner in which all register thermometers are still made. The very instrument just mentioned had a tube of large bore, and two indices more than three quarters ur an inch long, where half that size would certainly have been much better, and was farther mounted on a clumsy scale of boxwood, so that we confess it always surprised us that the in strument should act so well as it did. We ob served sometimes a liability in the mercury to change its position in the column of spirits, which made the two ends not always point to the same degree on the opposite scales, a defect to which the varying length of the column of mercury itself, by great extremes of temperature, may also con tribute.$ The indices are likewise liable to go wrong, and one of them constantly required the use of a very powerful horse shoe magnet after it had been for a short time in use. Were Six's thermometer, however, made with all the care and delicacy of which it is susceptible, it might, we have no doubt, be rendered an excellent instrument. It is perhaps better fitted than any yet contrived to measure the temperature of considerable depths of the ocean,* and likewise to investigate the de crease of heat in the higher regions of the atmos phere when applied to balloons.t A register thermometer by Dr. John Rutherford, is described in the Edinburgh Transactions, which, at least for the minimum, is decidedly superior to any other. That for showing the lowest tempera ture is shown at Plate DXXV. Fig. 1. It is a spirit thermometer, in the fluid of which is placed a little thread of enamel enlarged to a globule at each end seen at a, and this constitutes the whole instrument. From the cohesive power of the spirit confined within the tube, it does not suffer this little index to pass it without a very considerable shock ; it therefore carries it back to the lowest point of con traction and there leaves it, marking the greatest cold, and by expansion the index is not carried for ward. It is adjusted merely by inverting the instru ment till the index runs down to the end of the co lumn of spirit, where it stops. It is then laid in a horizontal position. The maximum thermometer, Fig. 2. is exactly the reverse of this, the index A being pushed forward by a column of mercury, and left at the point of greatest expansion. The index was originally of ivory, but from the friction with the interior of the empty tube, it was found advisa ble to make it of steel and adjust it with a magnet like Six's. Even this, however, does not prevent the index from getting entangled with the mercury, and Mr. Arlie of Edinburgh has more recently adopted the plan of placing naphtha above the mer cury, in which the index floats. This has somewhat altered the character of the instrument for simpli city, though we suspect it to be almost a necessary evil. The minimum thermometer, as we have al ready said, is almost unexceptionable; we have had long experience of it, and find it extremely little liable to go out of order. It is not yet, however, constructed with all the delicacy of which it is un doubtedly susceptible.