316 Royal Society. 



in turns of the dividing screw, for any degree x is ^, where R is 



the length of the step in which x occurs, and K,,. the equivalent 

 number of degrees. A table can then be constructed, showing the 

 lengths of each successive degi-ee, commencing from the point O, by 

 the aid of which the graduation may be performed. The scales ex- 

 tend to —75° Fahr. 



The time at my disposal was scarcely sufficient to test the ther- 

 mometers supplied to the Arctic Expedition so completely as I should 

 have wished. The mercurial thermometers were after their gradua- 

 tion compared incidentally at two or three different temperatures, 

 and found to agree generally to 0°" I Fahr. They were all placed in 

 melting ice, when it was found that four of them read exactly 32°, 

 the other three, viz. Nos. 34, 46, 47, were about 0°"1 too low. In 

 a few of these thermometers the column of mercury could be 

 readily broken : when this column was moved to different portions 

 of the scale, it was found to occupy precisely the same number of 

 divisions. This was the case with four of the instruments ; the other 

 three not having been tested in this way. 



The five spirit thermometers were compared at four different tem- 

 peratures with a standard mercurial thermometer. The comparison 

 at 0° being taken in ice and salt, is not very trustworthy. Their 

 errors were as follows : — 



+0-6 00 00 +1-7 -}-0-2 +0-50 



The numbers in the column " Mean of errors " seem to indicate 

 little error of a systematic nature. In the case of Nos. 2 and 7, the 

 index error is very large : this, it is believed, is owing to some of the 

 vapour of alcohol having become condensed in the upper portion of 

 the tube before the fixed points were determined, and having escaped 

 my notice ; in fact the greatest attention is required to avoid errors 

 from this source. These spirit thermometers cannot by any means 

 be considered as standard, although they are doubtless more trust- 

 worthy than most of those usually made. The limited time at my 

 command for the completion of the instruments, prevented the pos- 

 sibiHty of rectifying any blunders into which I might have fallen, 

 owing to my inexperience in such work, and the intricacy of the 

 problem. 



John Welsh, 

 Kew Observatory, April 21, 1852. 



