Introduction. 



XXXI 



Adie and Son were desired to make a thermometer, whose bulb rested in a cup in 

 a brass bar of the same dimensions as the magnet, and covered loosely by a small 

 brass cap.* The following comparisons were made of the indications of the two 

 thermometers, the box being in its original state, and the rise of temperature con- 

 siderable. 



Table 12. — Comparisons of the Thermometers with the Bulb free, and with the 



Bulb in a brass cup. 



GottinjTcn 



Thermometer. 



Difference. 



Mean Time. 











Koss. 



Adie. 





1844. 







d. h. 



° 



° 



° 



Jan. 2 21 



30 9 



307 



0-2 



22 



313 



310 



0-3 



23 



31 9 



31-5 



04 







33 9 



330 



09 



1 



38 9 



37-6 



1-3 



2 



423 



41 



1-3 



3 



44-7 



430 



1-7 



4 



45 6 



440 



1-6 



5 



45-9 



44 5 



1-4 



6 



46-0 



44-9 



11 



7 



461 



450 



11 



8 



45-9 



449 



1-0 



11 



45 



44-3 



0-7 



34. No observations have been made to determine the effect of the copper ring 

 or damper. It must, however, be small. 



During terms, and generally during magnetic disturbances, the temperature is 

 noted at the beginning of each hour ; the corrections for observations between the 

 hours are interpolated from those at the beginning of the hours. 



Absolute Horizontal Intensity. 



35. An observation of absolute horizontal intensity was made by Mr Eussell 

 March 26. 1842. It is given, page 66, with the particulars of reduction. 



As there was no extra declinometer, it was necessary to endeavour to eliminate 

 the changes of declination, by reversing the magnet. On account of the imperfec- 

 tions of this method, no other observations were made till 1843, when an extra in- 

 strument was obtained. 



* It was my original intention to enclose the bulb of the thermometer in a metallic capsule (from 

 which this differs little), but it was difficult to determine the thickness of the capsule. Professor 

 Lloyd, to whom I wrote, informed me that he had already adopted the precaution, having obtained a 

 brass bar of the dimensions of the magnet, containing a small iron cup with mercury, in which the 

 bulb of the thermometer was placed. 



