THERMOMETER AND PYROMETER. 



on nearly the same principle was con- 

 structed by Kraft : * but their con- 

 trivances are inferior to several others 

 of a later period, and do not require a 

 detailed notice. We shall therefore pro- 

 ceed to describe the most approved re- 

 gister thermometers. 



1. Lord Charles Cavendish commu- 

 nicated to the Royal Society different 

 forms of thermometers, intended to 

 register the maximum and minimum 

 temperature in the absence of the ob- 

 server, t His lordship's thermometer for 

 showing the maximum is represented in 

 %. 27. It consists of a cylindrical bulb, 



Fig. 29. 



Fig. 28. . Fig. 27- 



and a stem terminating in an open 

 capillary orifice, covered by a glass cap 

 or ball e completely closing the ther- 

 mometer. The bulb and part of the 

 stem are filled with mercury, the rise 

 and fall of which indicate the tempera- 

 ture in the usual way ; above the mer- 

 cury a portion of alcohol is intro- 

 duced, sufficient to fill the rest of the 



* Van Swinden, Comparaisondes Thermomfctres. 

 t Phil. Trans, vol. 1. for 175?. 



tube and a small part of the cap. When 

 the mercury rises it drives the spirit 

 before it into the cap e, from which it 

 cannot return while the instrument re- 

 mains erect ; and the deficiency of spirit 

 in the tube, on the subsiding of the 

 mercury, measured by a proper scale, will 

 show how much the maximum rise of 

 the thermometer exceeded its height at 

 the time of the observation. 



To prepare it for a fresh observation, 

 the thermometer is to be heated by the 

 hand until the spirit fills the whole 

 tube, which is then to be inclined so 

 that the spirit in the cap may cover the 

 capillary orifice : as the ball cools, the 

 spirit will drain into the tube thus in- 

 clined, and fill it as completely as before. 



Fig. 28 is a construction of the same 

 instrument, intended to obviate the in- 

 convenience of so weighty a bulb as that 

 filled with mercury must be. 



Lord C. adds a correction which 

 should be made on account of the 

 difference of expansion of mercury and 

 spirit, in computing the deficiency, if 

 this be measured by the same scale as 

 the ascent of the mercury : the degrees 

 computed by the column of spirit will 

 exceed those of the mercurial column 

 by i of a degree for every 10 of Fah- 

 renheit of difference between them. 



Fig. 29 is his lordship's minimum 

 thermometer. Its bulb, f of the ball d, 

 and part of the leg b, are to be filled 

 with spirit of wine ; from b to c is 

 occupied by a column of mercury, and 

 about i of d contains a portion of this 

 fluid ; a little alcohol is likewise intro- 

 duced above the mercury before the 

 orifice of the tube at e is closed in the 

 usual manner. The mercury at c will, 

 when furnished with a proper scale, 

 indicate the present temperature in the 

 ordinary way ; but, when the spirit in 

 the bulb contracts by cold, the mercury 

 will rise in the short leg of the siphon 

 from b into the ball d, from which it 

 cannot get back into the tube b. This 

 will therefore occasion a deficiency of 

 mercury in that leg, which, measured by 

 a proper scale attached to the short leg 

 of the siphon, and subtracted from the 

 present height of the mercury in the 

 long leg, will show the lowest point to 

 which the thermometer had fallen 

 during the absence of the observer. 

 To prevent the mercury falling in too 

 large drops into the ball d, by which the 

 delicacy of the instrument would be 

 impaired, a solid but fine thread of glass 

 passes through the short leg to the 



