2<»<^ Dffiriptlon ^ a IhermomeUr whkh 



VIII. 



Defcription of a Thermometer^ which marks the ^reatejl Degree of Heat and Cold from one Time 

 of Ohfervation to another^ and may alfo regifter its own Height at every Infant. By ALEX- 

 ANDER Keith, Efq. F.R.S. and F.A.S. Edinburgh. * 



X HERMOMETERS have hitherto been defedlve for meteorological puVpofes, in fo 

 far as they only point out the degree of heat at the moment of infpefting them, but do not 

 rtiow what the difference of temperature has been from the time of one obfervation to that of 

 another. Nor has any inftrument been yet conftru£led, fo far as I have been able to learn, 

 vi^hich vi'ili record the intermediate degrees of heat. 



The ingenious Robert Hooke, in the end of the lafl: centur)', mentions his intention of 

 making a thermometer for the fame purpofe ; but it does not appear that it ever w^as executed ; 

 neither does he explain how it was to have been done. 



The thermometer invented by Mr. James Six,/as defcribed in* the yzd volume of the Philo- 

 fophical TranfaSiions of the royal fociety of London, is made to fhow its greateft rife or fall 

 from one period of obfervation to another. This is done by means of two fmall pieces of 

 black glafs which float on two different furfaces of mercury, within two glafs tubes hermeti- 

 cally fealed. Thefe floats, when raifed to their greatefl height, adhere to the fide of the 

 tube by means of a fpring of glafs, and become flationary, although the mercury falls. 



After the obferver has taken a note of the temperature, he, by a magnet held in his hand, 

 draws down the float to the furface of the mercury, in confequence of a fmall bit of fleel wire 

 inclofed in the float, and the inftrument is prepared for another obfervation. This is an in- 

 genious invention, but requires too delicate workmanfhip to be fit for common ufe ; befides 

 it cannot be made to record the degrees of heat at intermediate periods. The thermometer 

 lately in vented by Dr. Rutherford of Balilifh, and defcribed in the third volume of the Tran- 

 faHions of this fociety, is alfo an ingenious contrivance ; but has the fame defeft of marking 

 only the extreme points to which the liquor has rifen or fallen in two feparate glafs tubes- 

 Several years ago it occurred to me, that an air thermometer might be ufed for the pur- 

 pofes required, provided the weight of the atmofphere could be excluded, or a counter ba- 

 lance formed to it ; and as the whole inftrument could be made to rife or fall by the temperature 

 of the atmofphere alone, it miglit be adapted to a piece of clock-work, which would record 

 the degrees of heat at every inftant through the year; and accordingly I read to this fociety 

 a defcription of the inftrument. But having formed another inftrument of a more fimple 

 conftrudtion to anfwer the fame purpofe, 1 beg leave to give a defcription of it. 



A B is a tube about fourteen inches long (pi. XII.) and three fourths of an inch caliber, of 

 thin glafs, fealed or clofed at top. To the bottom, which is bent upwards, there is joined a 

 glafs tube feven inches long, and four tenths of an inch caliber, open at top. The tube 



• Edin. Tranf. vol. IV. 



AB 



