THERMOMETER AND PYROMETER. 



33 



The sensibility of the instrument is re- The height of the instrument, from 

 presented as very great, when compared which this description is drawn, is 3 



to a mercurial thermometer ; and it is 

 applicable to such purposes as ascer- 

 taining the temperature of a vacuum, 

 which the mercurial thermometer is able 

 to do less accurately, because of the di- 

 latability of its bulb by the removal of 

 pressure. 



inches, including the feet, which are half 

 an inch ; the diameter of the helix is 

 rather less than T 3 o ; and its length is 

 1 \ ; the diameter of the graduated circle 

 is 2 inches inside, and its breadth \. 



1 9. The instrument delineated in fig. 

 26, from one now before us, is a 



Fig. 26. 



beautiful instrument of the same kind, 

 the work of the Parisian artist, Fre- 

 derick Houriet, which appears to be 

 little known in this country. It is of 

 the size of a thin ordinary watch, with a 

 dial A divided according to the centi- 

 grade scale. The mechanism is covered 

 by a thin plate of metal, which opens 

 like a hunting watch ; and the instru- 

 ment is so delicate as to move, in less 

 than a minute, after it is laid on the 

 hand, at an ordinary temperature of 

 GOT. 



The pyrometric piece is the bent 

 compound bar a, b, a, b, composed of 

 a plate of steel on the side a, and of 

 another of brass on the side b, united 

 together into one bar. The steel plate 

 is Jo inch in thickness, and the brass 

 twice as much, forming a bar 9.5 

 inches in length, and about } inch in 

 depth. One extremity is firmly secured 

 to the frame at c: the rest of it is 

 free, bent up for the convenience of 

 size, and secured against any accidental 

 injury from rude handling by passing 

 between two steel studs h, h. Its free 

 extremity is terminated by a plate of 

 steel d screwed to it, and projecting . 3 

 inch beyond it, to press against the 

 short arm e of the lever//. The long 

 arm of the lever ends in an arch- head 

 with thirty teeth, that play in the teeth 

 of a small wheel g with twenty-two 

 teeth, which is fixed on the axis of the 

 slender hand. The lengthening of the 

 bar pushes the short arm of the lever, 



and the arch-head moves over a space 

 proportional to the difference in length 

 of the arms of the lever. How this 

 motion is communicated in an increased 

 ratio to the index is obvious from the 

 construction. Under the cock i, which 

 supports the common axis of the index, 

 and g, is a spiral spring of flattened 

 gold wire, intended to bring back the 

 index when the contraction of the bar 

 allows it, and to retain the piece d in 

 contact with the short arm of the le- 

 ver e. The instrument is adjusted by 

 means of a steel screw k working in a 

 small tube, which perforates the end 

 piece d. 



The whole instrument is most deli- 

 cately made, and it accords in its indi- 

 cations with a mercurial centigrade 

 thermometer, with which it has been 

 carefully compared ; forming one of the 

 most elegant metalline thermometers 

 hitherto described. 



CHAPTER III. 



History and Construction of Register 

 Thermometers. 



THE original suggestion of a thermo- 

 meter which could register its own indi- 

 cations in the absence of the observer, is 

 due to the celebrated John Bernoulli, 

 who describes such an instrument in a 

 letter to Leibnitz ; * and an instrument 



* Leibnitzii et Bernoulli Commercium Philosoph. 

 et Mathcmat. 



