160 Remarks on Self-registering Thermometers. 



pear to have been the first registering ones described ; since 

 that of Six, which acts by indices, was not described in the 

 Philosophical Transactions for twenty-five years afterwards, in 

 1782. I must first notice the construction of Lord Charles 

 Cavendish'^s thermometer, which had the merit of superseding 

 the necessity of a common attached thermometer, which is re- 

 quired in the construction of Mr Blackadder and Mr King. 



A mercurial thermometer had the end of the tube drawn 

 to a capillary orifice, and was capped by a small glass recep- 

 tacle, exactly as represented in vol. ix. plate ii. fig. 7- and 9. of 

 this Journal ; above the mercury some alcohol was introdu- 

 ced into the tube, which of course was expelled into the glass 

 cistern through the capillary opening as the temperature rose, 

 and, as it could not draw it back when the temperature de- 

 clined, a space was left in the upper part of the tube, measured 

 by a descending scale of degrees, which gave the maximum 

 that had occurred since the last observation, when added to 

 the present temperature indicated by the height of the mercu- 

 ry in the tube, which never rises so high as to be expelled by 

 heat. 



This description, it will be observed, corresponds almost 

 precisely with that given by Mr King, and on the defects of 

 which I formerly made some remarks : my objection, relative 

 to the uncertainty of a fall of a drop of mercury from the 

 orifice, I find was expressed almost verbally in the same way by 

 Lord C. Cavendish,* in describing another of his thermometers, 

 where the capillary termination could not be so conveniently 

 employed, and which he proposed to rectify by inserting a glass 

 thread into the narrowest part of the tube, an expedient more 

 ingenious than practicable. This was employed in the mijii- 

 num thermometer, where the mercury fell into a globe be- 

 tween the tube and the real bulb, placed at the upper bend of 

 a syphon-shaped thermometer ; but the construction of this 



• *' If no farther contrivance was used, the mercury would fall into the 

 ball in large drops, which would make the instrument less accurate ; for 

 the thermometer's beginning to rise immediately after a drop has fallen, or 

 just as it is going to fall, (in which case it will return back to the tube,) 

 will make a difference of such part of a degree nearly as that drop an- 

 swers to." 



i 



