696 



THERMOMETER. 



instrument had, from its size, si ill greater defects I column of mercury may obviously be made con- 



for any delicate purposes, and has since been much 

 reduced. 



A register thermometer by Dr John Rutherford, 

 is described in the Edinburgh Transactions, vol. 

 iii., which, at least for the minimum, is decidedly 

 superior to any other. That for allowing the lowest 

 temperature is shown in Plate LXXXIX. fig. 14. 

 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 

 tin' 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 1 o 

 the lowest point of contraction, and there leaves it, 

 marking the greatest cold, and by expansion the 

 index is not carried forward. It is adjusted merely 

 by inverting the instrument till the index runs 

 down to the end of the column of spirit, where it 

 stops. It is then laid in a horizontal position. 

 The maximum thermometer, fig. 15, 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 advisable 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 

 Adie of Edinburgh has adopted the plan of placing 

 naphtha above the mercury, in which the index 

 floats. This has somewhat altered the character 

 of the instrument for simplicity, though we suspect 

 it to be almost a necessary evil. 



A contrivance intermediate between Six's and 

 Rutherford's has been proposed by Dr Trail. It is the 

 minimum thermometer of Rutherford, with a short 

 piece of mercury A (fig. 11.) introduced above 

 the index a, and the spirit again covering the mer- 

 cury contains another index b. It is manifest that 

 the mercury, by pushing back and forward these 

 indices, marks the two extremes 5 being of steel, 

 gilded by means of galvanism, they are brought 

 back by the magnet. : 



Mr Keith of Ravelstone has described a thermo- 

 meter which, by a mechanical contrivance, registers 

 for any given time, in a continuous form, the flux 

 of atmospheric temperature. (See Edinburgh 

 Transactions, vol. iv.) This mechanical invention 

 is very elegant, but has never met with, and is not 

 calculated for, general adoption. 



Le Chevalier has proposed lately a plan of regis- 

 tration sufficiently simple in its idea. It is the 

 adaptation of an index to the metallic thermo- 

 meters which have been described in another part 

 of this article. The difficulty, however, is to find 

 a method sufficiently delicate not to impede the 

 motion of these most susceptible instruments. No 

 plan has been given for this. 



Besides the registration of the greatest heat and 

 cold, thermometers have been proposed to record 

 the temperature at any given epoch. Mr Keith's 

 answers this purpose, but one more specifically 

 for this end has been proposed by Mr H. H. Black- 

 adder. (Edin. Trans, vol. iv.) He has a thermo- 

 meter A, fig. 16. constructed exactly similar to 

 Cavendish's maximum, only that it contains mer- 

 cury alone, and breaking the termination sharply- 

 over, fixes it into the case b, without bringing it to 

 a capillary termination, which we think a manifest 

 defect. When held with the bulb A lowest, the 



tiniious; but if by a piece of clock-work, the posi- 

 tion be made horizontal, as shown in the figure. 

 the column will be interrupted, and form u mark 

 of the height of the mercury at that moment, when 

 compared with the attached common thermometer 

 B. But if the temperature were to rise after the 

 given moment, the mercury would obviously lie ex- 

 pelled from the end of the tube : to avoid which 

 two hair pencils are contrived, at the moment of 

 immersion, to drop an evaporable fluid on tlie 

 bulbs, until they are observed, when by adding the 

 degrees vacant in tube Aft to the height of tin- 

 thermometer B, we have the temperature required 

 for the given epoch. By terminating the tube of 

 the thermometer A, in the manner shown i" 

 6, it is obvious that the same effects will be obtained 

 by a mere rotatory motion of the instrument, coin- 

 ciding with the axis of the tube. The complexity 

 of its construction combining a double thermo- 

 meter, a time piece, and an evaporating apparatus 

 is such as to prevent its adoption beyond a very 

 confined sphere. 



We must now say a word or two on a modifica- 

 tion of the common thermometer in its most 

 original form, which has been termed the differen- 

 tial thermometer. Professor Sturmius of Altdorff 

 was the inventor of this modification of the thermo- 

 meter. Professor Leslie and Count Rumlbrd both 

 revived it in 1804, under the titles of Differential 

 Thermometer and Thermoscope, one form of which 

 is shown in fig. 13. Its principle consists merely 

 in showing the difference of temperature between 

 two media, in which the balls A and B are placed, 

 and these being filled with air, the excess of ex- 

 pansion of air in the one above the other is marked 

 by the motion .of the coloured fluid which fills the 

 greater part of the bent stem, the height of which, 

 h, is referred to a scale attached. As it is strictly 

 a thermometer of differences, it refers merely to 

 the relative elasticities of the air in the two balls, 

 and being hermetically sealed, is free from the 

 common defects of the air thermometer. The 

 most important application of the differential 

 thermometer to practical meteorology is the hygro- 

 meter invented by Sir John Leslie, on the Hut- 

 tonian principle, of the difference between the tem- 

 perature of a dry and moistened surface, a ease 

 exactly fitted for the interposition of the differential 

 plan. The differential thermometer has been made 

 the basis of several other instruments by professor 

 Leslie, adapted to meteorology. 



Comparative Scales of Thermometers. A fertile 

 cause of error in estimating and comparing the state- 

 ments of temperature, is the very different man- 

 ner in which they are made by scientific men of 

 different nations. Wherever the English language 

 prevails, the graduation of Fahrenheit is generally 

 preferred. By the German authors Reaumur is 

 used ; and the French have, within a few years, 

 decided to adopt that of Celsius, a Swedish philo- 

 sopher, calling it thermometre centigrade. The 

 Russians still use the graduation of De Lisle. The 

 two remarkable temperatures of the boiling and the 

 freezing of water are thus expressed by the several 

 thermometers mentioned : 



Fahr. 

 212 

 32 



Centig. 



100 







Reanm. 

 80 

 



De Lisle 





 150 



Boiling point, 

 Freezing point, 



So that the number of degrees of each, included 

 between these two points in each, is 180 Fahr., 



