22.-. 



THERMOMETER. 



THERMOMETER. 



220 



instrument is intended to mark very low temperatures, it is desirable 

 to graduate it by means of two fixed points, namely, the freezing point 

 of water (32) and the freezing point of mercury ( 39). Professor 

 Miller has also proposed a third fixed point, namely, the fusion of solid 

 carbonic acid when it shall have been determined. The very low 

 artificial temperatures that have been recorded must not be insisted 

 upon, as the law of contraction of alcohol (which seems to form a con- 

 verging series) is not yet accurately known, and upon it the indications 

 of the spirit thermometer must of course depend. Even the low 

 natural temperatures which are sometimes recorded, are not always 

 trustworthy. During the severe cold of the winter of 1860-61, some 

 very low readings have been proved to be erroneous, in consequence of 

 the distillation of a portion of the spirit in the minimum thermometer 

 into the further extremity of the horizontal stem. It has been shown 

 that such a thermometer requires to be tilted from time to time to 

 connect this portion of alcohol with that in the stem, unless indeed the 

 new make come into general use, in which the end of the stem is 

 turned upwards, and the stem itself a little raised in the frame. 



Regiiter Thermometer*. It is of great importance in meteorology 

 that the observer should be able to ascertain the highest or lowest 

 point of a thermometer scale at which the column 

 of mercury may have stood during his absence ; 

 and a very large number of contrivances have 

 been made to obtain this end. Of these, one, 

 which is not quite gone out of use, was invented 

 by Mr. Six, whose name the instrument bears, and 

 is described in the ' Philosophical Transactions ' 

 fur 17S2. We retain it in this place, as it shows 

 the principle of construction in a very large num- 

 ber of register thermometers. Six's contrivance 

 consists of a long tube bent so as to form three 

 parallel branches, A, B, and c : the part A is, an 

 elongated bulb, and the rest of the tube has a 

 capillary bore. The lower portion, b, contains 

 mercury, which rises in B and c to certain points, 

 as a and r, and the bulb is filled with spirit of 

 wine, which passing over the bend at d, descends 

 to the upper extremity of the mercury in B : above 

 c the branch c is also filled with spirit to near the 

 upper extremity, which is hermetically sealed. 



Two small indices of steel coated with glass, 

 which are represented at m and n, are introduced 

 in the branches B and c : these are capable of being 

 forced upwards by the rising of the column of 

 mercury in either tube, and they have about them 

 a fine wire or a bristle or a thread of glass ; so 

 that they will remain stationary where they happen 

 to be when the heads a and c of the columns 

 recede from them. Their lower extremities con- 

 sequently indicate the points at which the ends 

 of the columns may have stood before such 

 recession. 



It is evident that the expansion of the spirit in 

 A by increments of heat will cause that which is 

 in B to press down the column of mercury in that branch and force up 

 the extremity c, moving the index before it, while by its friction the 

 index > is prevented from descending. On the other hand, the con- 

 traction of the spirit in A allows the elasticity of the air in the ball D 

 to force the column in c downwards, the index n remaining at the 

 highest point to which it had been previously raised ; the mercury in 

 B then rises up, carrying the index m before it, till an augmentation 

 of temperature causes the spirit in A to expand, and again force the 

 mercury in B downwards. 



The (graduations on the scale belonging to branch c are numbered 

 upwards ; while the graduations on the scale belonging to B are num- 

 bered downwards. The points a and c should always indicate the 

 same degree on the two scales ; and by means of a magnet the indices 

 m and n may be brought down to those points : from thence after- 

 wards the former ascends by a decrease, and the Litter by an increase of 

 temperature. 



This instrument ia apt to get out of order, and has given way 

 to simpler forms. The maximum thermometer, as now generally 

 made, is a mercurial thermometer with a horizontal stem. Within the 

 < a small piece of steel wire, which is pushed forward by the 

 expanding mercury, and is left stationary when the mercury begins to 

 contract and recede, thus marking the highest temperature in the 

 absence of the observer. The instrument can be reset for another 

 observation, by tilting it, or by applying a magnet to the outside 

 of the stem. The minimum thermometer is arranged like the maxi- 

 mum, only it contains coloured alcohol, the index being a small 

 piece of enamel terminating in a bead, and resting just below the sur- 

 face of the liquid column. As the liquid contracts by cold it carries 

 the index with it by capillary adhesion, but as soon as the liquid begins 

 to expand, it passes by the enamel, and leaves it at the lowest point to 

 which the liquid h:id descended, thus marking the minimum temper- 

 ature. To reset the instrument it may be tilted a little forward, or 

 the enamel may contain a piece of iron wire, which can be acted on by 

 a magnet on the outside. Another form of maximum thermometer, 



ARTS AND SCI. DIV. VOL. VIII. 



Six's BcgUtcr 

 Thermometer. 



by Negretti, is to contract the tube just above tlie bulb, so as to allow 

 the mercury to pass when it expands, but not to return when it con- 

 tracts. The maximum temperature is thus marked, and the instru- 

 ment can be reset by giving it a slight swing. The advantage of this 

 instrument is that there is no index to get out of order, but there is 

 some trouble in determining the corrections necessary to bring its 

 readings into unison with a standard thermometer. Professor Phillips 

 has a maximum thermometer, in which a portion of the mercurial 

 column is detached from the rest by means of a small bubble of air, 

 which detached portion remains stationary in the tube when the 

 mercury in the bulb contracts, and thus marks the highest reading. 

 The instrument is reset by swinging or inclining it. A good minimum 

 thermometer may still be considered as a desideratum. Messrs. 

 Negretti and Zambra have contrived one of very iugeuious construc- 

 tion, in which a slender-pointed needle is brought down to the surface 

 of the mercury in the tube, the latter being of wide bore, and in a 

 vertical position. As the mercury contracts the needle descends with 

 it, but on rising the mercury presses the needle to the glass, and rises 

 up by the side of it, instead of raising it. There are difficulties in the 

 manufacture of this instrument, and also in the method of observing 

 it, and all we can say of it is, that it is under trial. The introduction 

 of photographic registration [SELF-REGISTERING INSTRUMENTS] in 

 meteorological observatories has greatly limited the necessity for 

 mechanical registration. Even the wet and dry bulb thermometers 

 [HYGROMETER] are now registered photographically. 



The Chromatic Thermometer is a contrivance by Sir D. Brewster, in 

 which a number of superposed plates of glass exert under differences 

 of temperature a polarising action, which is estimated when the plates 

 are held in a beam of light polarised by reflection. 



Wollaston't Thermometer, for determining heights by variations 

 in the boiling point of water, is described under BOILING OP LIQUIDS. 



Differential Thermometer. This instrument, which was invented by 

 M. Sturmius, of Altdorf, before the year 1670, and was revived by 

 Professor Leslie, in 1804, consists of two ther- 

 mometer tubes, terminating, at one extremity s\ 

 of each, in a hollow glass ball, and containing v, r) 

 coloured sulphuric acid : the opposite extremities 

 are united by the flame of a blow-pipe, and an 

 enlargement of the bore is made at the place of 

 junction. The tube is then bent so as to form 

 three sides of a rectangle, the two balls, which 

 are of equal diameter, forming the upper extre- 

 mities of two sides ; and the instrument is on a 

 stand with the branches of the tube in vertical 

 positions. When the temperature of the air in 

 the two balls is the same, the acid occupies one 

 side and the base, and rises a little way up the 

 other side of the rectangle. To the latter side 

 is attached a graduated scale, with the zero of 

 which the upper extremity of the acid in that 

 branch should coincide. In the event of this Diffel . entia i Thcrmo . 

 adjustment being deranged, it may be restored meter< invcnte<1 b 

 by causing a small quantity of air to pass from M. sturmins of Altdorf. 

 one ball to the other, which is done simply by 

 the warmth of a hand applied to that ball from whence the air is to lie 

 driven. 



The variation^ of temperature in the apartment will evidently have 

 no effect on the instrument, since the action upon the two balls will 

 be equal : but if one ball alone be heated, the rise of the acid in tho 

 other will immediately indicate the difference between the tempera- 

 tures of the media about the two balU by the excess of the expansion 

 of the air in one ball above the expansion in the other. The delicacy 

 of the instrument is such that the least difference of temperature i i 

 immediately made sensible by the movement of the acid. 



Radiating Thermometer, or A<: For the purpose of 



measuring the intensity of solar or terrestrial radiation, an instrument, 

 called an ACTINOMETEU, has been contrived, for the use of which we 

 must refer to that head ; but it may be remarked that an approxima- 

 tion to the measure of solar radiation may be obtained by simply 

 exposing a register thermometer with a blackened ball to the dirvrt, 

 action of the sun's rays. [RADIATION.] The thermometer should be 

 placed a few inches above the ground, and be screened from currents 

 of air; and the graduations should be made on the stem of the thermo- 

 meter, in order to avoid the errors arising from the expansion or 

 warping of the scale. The force of terrestrial radiation may bu 

 measured by the minimum temperature of a register thermometer, 

 whose ball is placed in tho focus of a concave mirror : the face of the 

 mirror is to be turned towards the face of the sky, but away from the 

 rays of the sun. 



Since the establishment of the Kew Observatory, the construction of 

 meteorological instruments has received the earnest attention of com- 

 petent minds. Under BAROMETEB will be found the method by which 

 the Kew standard was formed by Mr. Welsh, the late superintendent. 

 He has also given directions for the construction and comparison of 

 thermometers ; his Report, presented to the Royal Society in 1852 

 (and printed not in the ' Transactions' but in the ' Abstracts of tho 

 Papers,' &c.,*), being founded in great measure on the plan proposed 

 Sec also British Association Report, 1853. 



Q 



