440 Mr BLACK ADDER'S Description of a new 



XXXI. Description of a new Register Thermometer, without any 

 Index ; the principle being applicable to the most deli- 

 cate Mercurial Thermometers. By H. H. BLACKADDER, 

 Esq. F. R. S. E. 



(Read April 17. 1826.J 



ON a former occasion, I had the honour of describing and ex- 

 hibiting to this Society a new Registering Thermometer, by 

 means of which the atmospheric temperature may be ascertained 

 at any given instant during absence. In the construction of 

 the instrument then described, a sliding index within the tube 

 is indispensable ; and, whenever such an index is employed, the 

 diameter of the tube, and consequently that of its bulb, must be 

 such as to render the instrument defective, when great accuracy 

 and attention to minute fractions is requisite, such as in barome- 

 trical measurements, and various delicate experiments. Besides, 

 though an instrument made with such an index may be so con- 

 structed as to perform with great accuracy, still, inasmuch as it 

 is a complication, it is defective, and is more or less liable to er- 

 ror, as its construction may have been more or less perfect. 



The instrument which I now mean to describe, is free from 

 all such objections, as no index of any description is requisite, 

 and it may be made of two of the most delicate mercurial ther- 

 mometers, both tubes being attached to the same slip of ivory, 

 but with a separate scale for each. 



One of the tubes, a, Plate XXII. Fig. 1. is hermetically seal- 

 ed as usual, and the scale also is divided and numbered in the 

 usual manner. The other tube d, is not hermetically sealed, but 

 left open at its upper extremity, which must be made flat and 

 smooth. This in general is easily and at once effected, by 

 making a small scratch with a sharp-edged file, previous to break- 



