Fig. 1. A B C D E
Fig. 2. A B C D E
Fig. 3. A B C D
Fig. 4. A B C D
Fig. 7. A F L H L'
Fig. 8. F K L G D
Fig. 5. i h c e d g a
Fig. 6. b l k m
Fig. 9. A B E F I O H D C 120° 110 100 90 80 70 60 50 40 30 20 10 0 7° 60 50 40 30 20 10 0 <table> <tr> <th></th> <th>II</th> <th>III</th> <th>IV</th> <th>V</th> <th>VI</th> <th>VII</th> <th>VIII</th> <th>IX</th> <th>X</th> <th>XI</th> <th>XII</th> <th>XIII</th> <th>XIV</th> <th>XV</th> <th>XVI</th> <th>XVII</th> </tr> <tr> <td>I</td> <td>Fahrenheit</td> <td>Florence</td> <td>Paris</td> <td>DelaHire</td> <td>Amontons</td> <td>Polemi</td> <td>Reaumur</td> <td>De l'Isle</td> <td>Crucequins</td> <td>R. Society Newton</td> <td>Fowler</td> <td>Hales</td> <td>Edinburgh</td> <td>Celsius</td> <td>De Luc</td> <td>or Reaumur</td> </tr> <tr> <td>108</td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> </tr> <tr> <td>104</td> <td>80</td> <td>40</td> <td>60</td> <td>1040</td> <td>90</td> <td>1250</td> <td>60</td> <td>40</td> <td>32</td> <td>104</td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> </tr> <tr> <td>100</td> <td>80</td> <td>40</td> <td>60</td> <td>1040</td> <td>90</td> <td>1240</td> <td>60</td> <td>23</td> <td>100</td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> </tr> <tr> <td>96</td> <td>100</td> <td>90</td> <td>59</td> <td>1230</td> <td>12</td> <td>22</td> <td>35</td> <td>28</td> <td>96</td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> </tr> <tr> <td>92</td> <td>70</td> <td>80</td> <td>58</td> <td>1030</td> <td>100</td> <td>1220</td> <td>11</td> <td>50</td> <td>21</td> <td>92</td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> </tr> <tr> <td>88</td> <td>90</td> <td>80</td> <td>58</td> <td>1210</td> <td>0</td> <td>11</td> <td>20</td> <td>30</td> <td>24</td> <td>88</td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> </tr> <tr> <td>84</td> <td>90</td> <td>80</td> <td>58</td> <td>1200</td> <td>10</td> <td>50</td> <td>19</td> <td>84</td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> </tr> <tr> <td>80</td> <td>60</td> <td>80</td> <td>57</td> <td>110</td> <td>1190</td> <td>10</td> <td>9</td> <td>40</td> <td>18</td> <td>25</td> <td>20</td> <td>76</td> <td></td> <td></td> <td></td> <td></td> </tr> <tr> <td>76</td> <td>30</td> <td>70</td> <td>51</td> <td>1180</td> <td>20</td> <td>8</td> <td>17</td> <td>72</td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> </tr> <tr> <td>72</td> <td>70</td> <td>56</td> <td>1020</td> <td>120</td> <td>1160</td> <td>7</td> <td>30</td> <td>16</td> <td>68</td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> </tr> <tr> <td>68</td> <td>50</td> <td>60</td> <td>1150</td> <td>20</td> <td>6</td> <td>20</td> <td>15</td> <td>64</td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> </tr> <tr> <td>64</td> <td>60</td> <td>55</td> <td>50</td> <td>1140</td> <td>40</td> <td>5</td> <td>10</td> <td>14</td> <td>12</td> <td>60</td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> </tr> <tr> <td>60</td> <td>40</td> <td>50</td> <td>54</td> <td>130</td> <td>1130</td> <td>4</td> <td>0</td> <td>13</td> <td>56</td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> </tr> <tr> <td>56</td> <td>20</td> <td>49</td> <td>1010</td> <td>1120</td> <td>50</td> <td>3</td> <td>12</td> <td>10</td> <td>52</td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> </tr> <tr> <td>48</td> <td>40</td> <td>40</td> <td>53</td> <td>140</td> <td>1100</td> <td>2</td> <td>10</td> <td>11</td> <td>44</td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> </tr> <tr> <td>44</td> <td>30</td> <td>48</td> <td>1000</td> <td>1090</td> <td>70</td> <td>1</td> <td>20</td> <td>10</td> <td>40</td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> </tr> <tr> <td>36</td> <td>30</td> <td>52</td> <td>1080</td> <td>1070</td> <td>0</td> <td>30</td> <td>0</td> <td>9</td> <td>36</td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> </tr> <tr> <td>32</td> <td>20</td> <td>51</td> <td>47</td> <td>1060</td> <td>80</td> <td>0</td> <td>8</td> <td>6</td> <td>32</td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> </tr> <tr> <td>28</td> <td>20</td> <td>50</td> <td>160</td> <td>1050</td> <td>90</td> <td>40</td> <td>28</td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> </tr> <tr> <td>24</td> <td>10</td> <td>10</td> <td>160</td> <td>1040</td> <td>50</td> <td>5</td> <td>4</td> <td>24</td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> </tr> <tr> <td>20</td> <td>10</td> <td>10</td> <td>990</td> <td>1030</td> <td>100</td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> </tr> <tr> <td>16</td> <td>0</td> <td>10</td> <td>170</td> <td>1020</td> <td>110</td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> </tr> <tr> <td>12</td> <td>0</td> <td>10</td> <td>170</td> <td>1010</td> <td>120</td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> </tr> <tr> <td>8</td> <td>0</td> <td>0</td> <td>49</td> <td>1600</td> <td>120</td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> </tr> <tr> <td>4</td> <td>0</td> <td>0</td> <td>48</td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> </tr> <tr> <td>0</td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> </tr> </table>
W. Train Sculpt Equation of the Boiling Point.
<table> <tr> <th>Barometer.</th> <th>Equation.</th> <th>Difference.</th> </tr> <tr> <td>31.0</td> <td>+ 1.57</td> <td>0.78</td> </tr> <tr> <td>30.5</td> <td>+ 0.79</td> <td>0.79</td> </tr> <tr> <td>30.0</td> <td>0.00</td> <td>0.80</td> </tr> <tr> <td>29.5</td> <td>- 0.80</td> <td>0.82</td> </tr> <tr> <td>29.0</td> <td>- 1.62</td> <td>0.83</td> </tr> <tr> <td>28.5</td> <td>- 2.45</td> <td>0.85</td> </tr> <tr> <td>28.0</td> <td>- 3.31</td> <td>0.86</td> </tr> <tr> <td>27.5</td> <td>- 4.16</td> <td>0.88</td> </tr> <tr> <td>27.0</td> <td>- 5.04</td> <td>0.88</td> </tr> </table>
The numbers in the first column of this table express heights of the quicksilver in the barometer in English inches and decimal parts; the second column shows the equation to be applied, according to the sign prefixed, to 212° of Bird's Fahrenheit, to find the true boiling point for every such state of the barometer. The boiling point for all intermediate states of the barometer may be had with sufficient accuracy, by taking proportional parts, by means of the third column of differences of the equations. See Philosophical Transactions, vol. lxiv. art. 30.; also Dr Maskelyne's Paper, vol. lxiv. art. 20.
In the following table we have the result of fifteen different observations made by Sir George Shuckburgh compared with the result of M. de Luc's rules.
<table> <tr> <th>Height of the Barometer reduced to the same temperature of 50°.</th> <th>Mean Boiling Point by Observation.</th> <th>Boiling Point by De Luc's Rules.</th> <th>Height of Barometer.</th> <th>Boiling Point by Observation.</th> <th>Boiling Point by De Luc's Rules.</th> </tr> <tr> <td>Inch.</td> <td>0</td> <td>0</td> <td>Inch.</td> <td>0</td> <td>0</td> </tr> <tr> <td>26.498</td> <td>207.07</td> <td>208.54</td> <td>30.008</td> <td>213.22</td> <td>213.47</td> </tr> <tr> <td>27.241</td> <td>208.64</td> <td>208.84</td> <td>30.207</td> <td>213.58</td> <td>213.79</td> </tr> <tr> <td>27.954</td> <td>209.87</td> <td>210.03</td> <td>30.489</td> <td>214.15</td> <td>214.23</td> </tr> <tr> <td>28.377</td> <td>210.50</td> <td>210.81</td> <td>30.763</td> <td>214.37</td> <td>214.66</td> </tr> <tr> <td>28.699</td> <td>211.27</td> <td>211.34</td> <td>30.847</td> <td>214.83</td> <td>214.79</td> </tr> <tr> <td>28.898</td> <td>211.50</td> <td>211.67</td> <td>30.937</td> <td>214.96</td> <td>214.96</td> </tr> <tr> <td>28.999</td> <td>211.60</td> <td>211.85</td> <td>30.957</td> <td>214.96</td> <td>214.96</td> </tr> <tr> <td>29.447</td> <td>212.55</td> <td>212.74</td> <td></td> <td></td> <td></td> </tr> <tr> <td>29.805</td> <td>212.95</td> <td>213.15</td> <td></td> <td></td> <td></td> </tr> </table>
Sir George Shuckburgh has also subjoined the following general table for the use of artists in constructing the thermometer, both according to his own observations and those of M. de Luc.
<table> <tr> <th>Height of the Barometer.</th> <th>Correct. of the Boiling Point.</th> <th>Difference.</th> <th>Correct. according to M. de Luc.</th> <th>Difference.</th> </tr> <tr> <td>26.0</td> <td>7.09</td> <td>.91</td> <td>6.83</td> <td>.90</td> </tr> <tr> <td>26.5</td> <td>6.18</td> <td>.91</td> <td>5.93</td> <td>.89</td> </tr> <tr> <td>27.0</td> <td>5.27</td> <td>.90</td> <td>5.04</td> <td>.88</td> </tr> <tr> <td>27.5</td> <td>4.37</td> <td>.89</td> <td>4.16</td> <td>.87</td> </tr> <tr> <td>28.0</td> <td>3.48</td> <td>.89</td> <td>3.31</td> <td>.86</td> </tr> <tr> <td>28.5</td> <td>2.59</td> <td>.87</td> <td>2.45</td> <td>.83</td> </tr> <tr> <td>29.0</td> <td>1.72</td> <td>.87</td> <td>1.62</td> <td>.82</td> </tr> <tr> <td>29.5</td> <td>0.85</td> <td>.85</td> <td>0.80</td> <td>.80</td> </tr> <tr> <td>30.0</td> <td>0.00</td> <td>.85</td> <td>0.00</td> <td>.80</td> </tr> <tr> <td>30.5</td> <td>+ 0.85</td> <td>.85</td> <td>+ 0.79</td> <td>.79</td> </tr> <tr> <td>31.0</td> <td>+ 1.69</td> <td>.84</td> <td>+ 1.57</td> <td>.78</td> </tr> </table>
The Royal Society, fully apprised of the importance of adjusting the fixed points of thermometers, appointed a committee of seven gentlemen to consider of the best method for this purpose; and their report is published in the Phil. Transf. vol. lxvii. part ii. art. 37.
They observed, that though the boiling point be placed so much higher on some of the thermometers now made than on others, yet this does not produce any considerable error in the observations of the weather, at least in this climate; for an error of 1 1/2° in the position of the boiling point, will make an error only of half a degree in the position of 92°, and of not more than a quarter of a degree in the point of 62°. It is only in nice experiments, or in trying the heat of hot liquors, that this error in the boiling point can be of much importance.
In adjusting the freezing as well as the boiling point, the quicksilver in the tube ought to be kept of the same heat as that in the ball. When the freezing point is placed at a considerable distance from the ball, the pounded ice should be piled to such a height above the ball, that the error which can arise from the quicksilver in the remaining part of the tube not being heated equally with that in the ball, shall be very small, or the observed point must be corrected on that account according to the following table:
<table> <tr> <th>Heat of the Air.</th> <th>Correction.</th> </tr> <tr> <td>42°</td> <td>.00087</td> </tr> <tr> <td>52°</td> <td>.00174</td> </tr> <tr> <td>62°</td> <td>.00261</td> </tr> <tr> <td>72°</td> <td>.00348</td> </tr> <tr> <td>82°</td> <td>.00435</td> </tr> </table>
The correction in this table is expressed in 1000th parts of the distance between the freezing point and the surface of the ice: e. gr. if the freezing point stands seven inches above the surface of the ice, and the heat of the room is 62, the point of 32° should be placed 7 × 0.00261, or .018 of an inch lower than the observed point. A diagonal scale will facilitate this correction. The committee observe, that in trying the heat of liquors, care should be taken that the quicksilver in the tube of the thermometer be heated to the same degree as that in the ball; or if this cannot be done conveniently, the observed heat should be corrected on that account; for the manner of doing which, and a table calculated for this purpose, we must refer to their excellent report in the Phil. Trans. vol. lxvii. part ii. art. 37.
With regard to the choice of tubes, they ought to be exactly cylindrical. But though the diameter should vary a little, it is easy to manage that matter in the manner proposed by the Abbé Nollet *, by making a small portion of the quicksilver, e. gr. as much as fills up an inch or half an inch, slide backward and forward in the tube; and thus to find the proportions of all its inequalities, and from thence to adjust the divisions to a scale of the most perfect equality. The capillary tubes are preferable to others, because they require smaller bulbs, and they are also more sensible, and less brittle. The most convenient size for common experiments has the internal diameter about the 40th or 50th of an inch, about nine inches long, and made of thin glass, that the rise and fall of the mercury may be better seen.
The next thing to be considered, is of what number of degrees or divisions the scale ought to consist, and from what point it ought to commence. As the number of the divisions of the scale is an arbitrary matter, the scales which have been employed differ much from one another in this circumstance. Fahrenheit has made 180 degrees between the freezing and boiling water point. Amontons made 73, and Sir Isaac Newton only 34. There is, however, one general maxim, which ought to be observed: That such an arithmetical number should be chosen as can easily be divided and subdivided, and that the number of divisions should be so great that there shall seldom be occasion for fractions. The number 80 chosen by Reaumur answers extremely well in this respect, because it can be divided by several figures without leaving a remainder; but it is too small a number: the consequence of which is, that the degrees are placed at too great a distance from one another, and fractions must therefore be often employed. We think, therefore, that 160 would have been a more convenient number. Fahrenheit's number 180 is large enough, but when divided its quotient soon becomes an odd number.
As to the point at which the scale ought to commence, various opinions have been entertained. If we knew the beginning or lowest degree of heat, all philosophers would agree, that the lowest point of the thermometer ought to be fixed there; but we know neither the lowest nor the highest degrees of heat; we observe only the intermediate parts. All that we can do, then, is to begin it at some invariable point, to which thermometers made in different places may easily be adjusted. If possible too, it ought to be a point at which a natural well-known body receives some remarkable change from the effects of heat or cold. Fahrenheit began his scale at the point at which snow and salt congeal. Kirwan proposes the freezing point of mercury. Sir Isaac Newton, Hales, and Reaumur adopted the freezing point of water. The objection to Fahrenheit's lowest point is, that it commences at an artificial cold never known in nature, and to which we cannot refer our feelings, for it is what few can ever experience. There would be several great advantages gained, we allow, by adopting the freezing point of mercury. It is the lowest degree of cold to which mercury can be applied as a measure; and it would render unnecessary the use of the signs plus and minus, and the extension of the scale below 0°. But we object to it, that it is not a point well known; for few, comparatively speaking, who use thermometers, can have an opportunity of seeing mercury congealed. As to the other advantage to be gained by adopting the freezing point of mercury, namely, the abolition of negative numbers, we do not think it would counterbalance the advantage, to be enjoyed by using a well-known point. Besides, it may be asked, Is there not a propriety in using negative numbers to express the degree of cold, which is a negative thing? Heat and cold we can only judge of by our feelings: the point then at which the scale should commence, ought to be a point which can form to us a standard of heat and cold; a point familiar to us from being one of the most remarkable that occurs in nature, and therefore a point to which we can with most clearness and precision refer to in our minds on all occasions. This is the freezing point of water chosen by Sir Isaac Newton, which of all the general changes produced in nature by cold is the most remarkable. It is, therefore, the most convenient point, for the thermometers to be used in the temperate and frigid zones; we may say over the globe, for even in the hottest countries of the torrid zone many of the mountains are perpetually covered with snow.
The thermometers which are at present in most general use, are Fahrenheit's, De l'Isle's, Reaumur's, and Celsius's. Fahrenheit's is used in Britain, De l'Isle's in Russia, Reaumur's and the thermometer centigrade in France, and Celsius's, the same as the last named, in Sweden. They are all mercurial thermometers. For their description and the method of comparing them together, see Chemistry, No 198—201. See also Plate DXXXIV.
As in meteorological observations it is necessary to attend to the greatest rise and fall of the thermometer, attempts have been made to construct a thermometer which might register the greatest degree of heat, or greatest degree of cold, which took place during the absence of the observer. In 1757 Lord Charles Cavendish presented to the Royal Society of London a thermometer-thermometer in two different forms; the one contrived to mark the greatest degree of heat, and the other the greatest degree of cold.
The first consists of a glass tube AB, fig. 3, with a cylindrical bulb B at the lower end, and capillary at the top, over which there is fixed a glass ball C. The bulb and part of the tube are filled with mercury, the top of which shows the degrees of heat as usual. The upper part of the tube above the mercury is filled with spirit of wine; the ball C is also filled with the same liquor almost to the top of the capillary tube. When the mercury rises the spirit of wine is also raised, and falls into the ball C, which is so made that the liquor cannot return into the tube when the mercury sinks; consequently the height of the spirit of wine in the ball, added to that in the tube, will give the greatest degree of heat to which the thermometer has pointed since last observation. When a new observation is to be made, the instrument must be inclined inclined till the liquor in the ball cover the end of the capillary tube.
In this thermometer it is evident that the mercury must be affected by the weight and elasticity of the spirit of wine, and therefore it will not correspond to any of the common mercurial thermometers.
The thermometer for showing the greatest degree of cold is represented in fig. 4. by the crooked tube ABCD. This instrument is filled with spirit of wine, with the addition of as much mercury as is sufficient to fill both legs of the siphon, and about a fourth or fifth part of the hollow ball C. We are not told what the proportion of mercury was to that of spirit of wine. The degrees of heat are shown by the rise or fall of the mercury in the leg AB. The thermometer marks the greatest fall by means of the hollow ball C. When the mercury in the longer leg sinks by cold, that in the shorter will rise and run over into the ball C, from which it cannot return when the mercury sublides in the shorter and rises in the longer leg. The upper part of the shorter leg will therefore be filled with a column of spirits of a length proportional to the increase of heat; the bottom or lower surface of which, by means of a proper scale, will show how much the mercury has been lower than it is; which being subtracted from the present height will give the lowest point to which the mercury has fallen. That the thermometer may be fitted for a new observation, the mercury must be made to run back from the ball into the shorter leg, by inclining the tube and heating the ball.
In 1782 Mr Six proposed another self-registering thermometer. It is properly a spirit of wine thermometer, though mercury is also employed for supporting an index. a b, fig. 5. in a thin tube of glass 16 inches long, and \( \frac{3}{8} \)ths of an inch caliber: c d e and f g h are smaller tubes about \( \frac{1}{16} \)th of an inch caliber. These three tubes are filled with highly rectified spirit of wine, except the space between d and g, which is filled with mercury. As the spirit of wine contracts or expands in the middle tube, the mercury falls or rises in the outside tubes. An index, such as that represented in fig. 6. is placed on the surface, within each of these tubes, so light as to float upon it. k is a small glass tube \( \frac{3}{8} \)ths of an inch long, hermetically sealed at each end, and including a piece of steel wire nearly of its own length. At each end l, m, of this small tube, a short tube of black glass is fixed, of such a diameter as to pass freely up and down within either of the outside tubes of the thermometer e o or f h. From the upper end of the index is drawn a spring of glass to the fineness of a hair, and about \( \frac{1}{8} \)ths of an inch long; which being placed a little oblique, presses lightly against the inner surface of the tube, and prevents the index from descending when the mercury descends. These indexes being inserted one into each of the outside tubes, it is easy to understand how they point out the greatest heat or cold that has happened in the observer's absence. When the spirit of wine in the middle tube expands, it presses down the mercury in the tube h f, and consequently raises it in the tube e o; consequently the index on the left hand tube is left behind and marks the greatest cold, and the index in the right hand tube rises and marks the greatest heat.
In 1790 a paper was given in to the Royal Society of Edinburgh, describing two thermometers, newly invented, by Dr John Rutherford of Middle Bailiff; the one for registering the highest and the other for registering the lowest degree of heat to which the thermometer has risen or fallen during the absence of the observer. An account of them may be found in the third volume of the Transactions of the Society.
A new self-registering thermometer has been invented by Mr Keith of Ravellstone, which we consider as the most ingenious, simple, and perfect, of any which has hitherto appeared. Its simplicity is so great, that it requires only a very short description to make it intelligible.
AB, fig. 7. is a thin glass tube about 14 inches long Fig. 7. and \( \frac{1}{4} \)ths of an inch caliber, close or hermetically sealed at top. To the lower end, which is open, there is joined the crooked glass tube BE, seven inches long, and \( \frac{1}{8} \)ths of an inch caliber, and open at top. The tube AB is filled with the strongest spirit of wine, and the tube BE with mercury. This is properly a spirit of wine thermometer, and the mercury is used merely to support a piece of ivory or glass, to which is affixed a wire for raising one index or depressing another, according as the mercury rises or falls. E is a small conical piece of ivory or glass, of such a weight as to float on the surface of the mercury. To the float is joined a wire called the float-wire, which reaches upwards to H, where it terminates in a knee bent at right angles. The float-wire, by means of an eye at a, moves easily along the small harpichord wire GK. LL are two indexes made of thin black oiled silk, which slide upwards or downwards with a force not more than two grains. The one placed above the knee points out the greatest rise, and the one placed below it points out the greatest fall, of the thermometer.
When the instrument is to be prepared for an observation, both indexes are to be brought close to the knee H. It is evident, that when the mercury rises, the float and float-wire, which can be moved with the smallest force, will be pushed upwards till the mercury become stationary. As the knee of the float-wire moves upwards, it will carry along with it the upper index L. When the mercury again sublides, it leaves the index at the highest point at which it was raised, for it will not descend by its own weight: As the mercury falls the float-wire does the same; it therefore brings along with it the lower index L, and continues to depress it till it again becomes stationary or ascend in the tube; in which case it leaves the lower index behind it as it had formerly left the upper. The scale to which the indexes point is placed parallel to the slender harpichord wire. It may be seen more distinctly in fig. 8. fig. 8. That the scale and indexes may not be injured by the wind and rain, a cylindrical glass cover, close at top, and made so as exactly to fit the part GF, is placed over it.
The ingenious inventor has another improvement in contemplation, which, if upon trial it be found to answer, will make this thermometer as perfect as can be desired, provided there do not arise some errors from the variable pressure of the atmosphere. He proposes to adapt clock-work to this thermometer, in such a way as to register with the utmost precision the degrees of heat and cold for every month, day, and minute in the year. The principles on which this clockwork is to be formed we shall forbear to describe, hoping that the author himself, after his experiment has met with the success which we ardently wish, will favour the world with his own account of it.
The same ingenious gentleman has invented a self-registering barometer, upon the same principles with the self-registering thermometer. We have had the pleasure of seeing both; and are convinced that they will fully gratify the wishes of all who are engaged in meteorological studies. He is also in expectation of being soon able to produce an air-thermometer free from the defects of those which were formerly made, as he has found out a way of preventing it from being affected by the pressure of the atmosphere.
M. de Luc has described the best method of constructing a thermometer, fit for determining the temperature of the air, in the mensuration of heights by the barometer. He has also shewn how to divide the scale of a thermometer, so as to adapt it for astronomical purposes in the observation of refractions.
Mr Cavallo, in 1781, proposed the construction of a thermometrical barometer, which, by means of boiling water, might indicate the various gravity of the atmosphere, or the height of the barometer. But as he does not say that the instrument has been tried with the desired success, we forbear to describe it. Those who wish to know his ideas respecting it may consult the Philosophical Transactions, vol. lxxi. p. 524.
The thermometers hitherto described are very limited in their extent; they indeed point out to us the lowest degrees of heat which are commonly observed even in cold climates, but they by no means reach to those degrees of heat which are very familiar to us. The mercurial thermometer extends no farther than to 600° of Fahrenheit's scale, the heat of boiling mercury; but we are sure that the heat of solid bodies, when heated to ignition, or till they emit light, far exceeds the heat of boiling mercury.
In order to remedy this defect, Sir Isaac Newton, whose genius overcame those obstacles which ordinary minds could not approach, attempted by an ingenious experiment to extend the scale to any degree required. Having heated a mass of iron red hot, and exposed it to the cold air, he observed the time which elapsed till it became cold, or of the same temperature with the air; and when the heat so far decreased that he could apply some known measure (as a thermometer) to it, he observed the degrees of heat lost in given times; and thence drew the general conclusion, that the quantities of heat lost in given small spaces are always proportional to the heat remaining in the body, reckoning the heat to be the excess by which it is warmer than the ambient air. So that taking the number of minutes which it took to cool after it came to a determined point in an arithmetical progression, the decrements of the heat of the iron would be continually proportional. Having by this proportion found out the decrements of heat in a given time after it came to a known point, it was easy, by carrying upwards the same proportion to the beginning of its cooling, to determine the greatest heat which the body had acquired. This proportion of Sir Isaac's was found by Dr Martine to be somewhat inaccurate. The heat of a cooling body does not decrease exactly in proportion to that which the body retains. As the result of many observations, he found that two kinds of proportion took place, an arithmetical as well as the geometrical proportion which Sir Isaac Newton had adopted; namely, that the decrements of heat were partly proportional to the times (that is, that quantities of heat are lost in equal times), as well as partly in proportion to the remaining heat; and that if these two are added together the rule will be sufficiently accurate. By the geometrical proportion which Sir Isaac Newton adopted he discovered the heat of metals red hot or in infusion.
This method, so successfully pursued by Sir Isaac, was sufficient to form a scale of high degrees of heat, but was not convenient for practical purposes. Accordingly the late Mr Wedgwood invented a very simple thermometer which marks with much precision high and the different degrees of ignition from a dull red heat visible in the dark to the heat of an air-furnace. It consists of two rulers fixed upon a smooth flat plate, a little farther afield at the one end than at the other, leaving an open longitudinal space between them. Small pieces of alum and clay mixed together are made of such a size as just to enter at the wide end; and are heated along with the body whose heat we wish to determine. The earthy body contracts according to the degree of heat, so that when applied to the wide end of the gage, it will slide on towards the narrow end, less or more according to the degree of heat to which it has been exposed.
ABCD, fig. 9, is a smooth flat plate; and EF and GH two rulers or flat pieces, a quarter of an inch thick, fixed flat upon the plate, with the sides that are towards one another made perfectly true, a little farther afield at one end EG than at the other end FH: thus they include between them a long converging canal, which is divided on one side into a number of small equal parts, and which may be considered as performing the offices both of the tube and scale of the common thermometer. It is obvious, that if a body, so adjusted as to fit exactly at the wider end of this canal, be afterwards diminished in its bulk by fire, as the thermometer pieces are, it will then pass further in the canal, and more and more so according as the diminution is greater; and conversely, that if a body, so adjusted as to pass on to the narrow end, be afterwards expanded by fire, as is the case with metals, and applied in that expanded state to the scale, it will not pass so far; and that the divisions on the side will be the measures of the expansions of the one, as of the contractions of the other, reckoning in both cases from that point to which the body was adjusted at first.
I is the body whose alteration of bulk is thus to be measured. This is to be gently pushed or slid along towards the end FH, till it is stopped by the converging sides of the canal. See Chemistry, No 1412.
A very ingenious application of Fahrenheit's thermometer has been made by Mrs Lovi, glass-blower in Edinburgh, for ascertaining the temperature of compost dunghills, for regulating the temperature of hot-beds, and observing the changes of temperature in corn and haystacks when they are put up damp. This may be called an agricultural thermometer, and has been found of great use for the above purposes.
THERMOPYLÆ, in Ancient Geography, a narrow pass or defile, between the wash of the Sinus Malacus on the east, and steep mountains, reaching to Oeta, made dreadful by impassable woods, on the west; leading from Thessaly to Locris and Beotia. These moun- Theoppy-tains divide Greece in the middle, in the same manner as the Apennine does Italy; forming one continued ridge from Leucate on the west to the sea on the east, with thickets and rocks interspersed; that persons even prepared for travelling, much less an army encumbered with baggage, cannot easily find a commodious passage. In the valley verging towards the Sinus Malaccus, the road is only sixty paces broad; the only military way for an army to pass, if not obstructed by an enemy; and therefore the place is called Pyler, and by others, on account of its hot water, Thermopylae. Ennobled by the brave stand made by Leonidas and 300 Spartans against the whole army of Persia; and by the bold resolution of blind Euthyceus, choosing rather to fall there in fight, than return to Sparta, and escape the common danger. Famous also for the Amphidryones, the common council or states general of Greece, assembling there twice a-year, spring and autumn. For an account of the battle of Thermopylae at which Leonidas with a handful of men engaged the Persian army, see SPARTA.