86 Proceedings of Eoyal Society of Edinburgh. [jan. 6, 
with the dimensions commonly given to the instrument. It has 
also some other defects, among which may he mentioned difficulties 
as to the capillary surfaces of the mercury, want of flexibility or 
adaptability for various positions, and the proximity of the mano- 
metric column to the heated regions. 
The modiflcations which I have made in the construction of the 
air thermometer have a threefold object, one part of which is to 
improve on the accuracy of the instrument, and reduce to the 
minimum that is practicable the correction above referred to for the 
air expelled by heat from the thermometer bulb or air reservoir. 
A second object is to increase the range of the instrument by giving 
it a form in which the hard Bohemian glass can be used in the con- 
struction of the part to be heated. The third object is to make 
that part of the thermometer which is to be heated, and which, in 
the use of the instrument, must be put in position with other pieces 
of experimental apparatus, of such a form as to be easily handled. 
