Sikes 1 Self-Registering Thermometer. 85 



present made by the London glass-blowers, works without 

 much liability to derangement, after it has been safely sus- 

 pended in the place intended for its reception. The annexed 

 diagram is given in order to explain the principle on which 

 Sikes' self-registering thermometer is constructed : Briefly it 

 may be styled a spirit- thermometer, with a prolonged stem 

 ending in an inverted mercurial syphon, the two surfaces of 

 the mercury syphon working small indexes which register 

 the temperature. 



a a, is the thermometer bulb filled with alcohol. 



b b, the thermometer stem. 



ccdde, a prolongation of the thermometer stem. 



c c c c, the portion of the prolonged stem filled with mercury. 



d d, further prolongation of the stem filled with alcohol. 



e, a small bulb at the top filled with air or vapour of alcohol. 



//, two small indexes worked by the mercurial surfaces. 



The error in the indications of the instrument which I have 

 alluded to, arises in periods of extreme variations of tempera- 

 ture. It is occasioned by the great difference in the specific 

 gravities of mercury and of alcohol, and also from the mer- 

 cury not adhering to the tube as the alcohol does. In frosty 

 weather the mercury on the side b b stands highest, but in 

 warm weather the side d d is the higher ; the difference in 

 level often amounts to a hydrostatic pressure equal to one 

 pound per superficial inch. The part of the syphon occupied 

 by the mercury is coated with a film of alcohol, which connects 

 the alcohol of the thermometer a abb, with the alcohol in the 

 part d d. Now when there is a difference of pressure in the 

 arms of the mercurial syphon, alcohol passes slowly by a ca- 

 pillary process from the one side of the syphon to the other, 

 to restore the equilibrium of the mercury, and in doing so 

 destroys the accuracy of the instrument; for it is necessary 

 that the quantity of alcohol in the thermometric part a abb, 

 should be constant. This is the fault of Sikes' registering ther- 

 mometer ; in practise, both sides of the mercury exhibit the 

 same reading, yet when a mercurial thermometer is sus- 

 pended beside it, the indication is found 7 to 10 degrees 

 wrong. For this great error, there is no direct remedy, but 

 the following addition is a ready mode of making the instru- 



