196 



effect of a source of heat, it might perhaps he done by placing this 

 instrument near the source, alternately exposing it to the influence of 

 the calorific rays proceeding from the source, and intercepting these 

 by means of a screen. Owing to the peculiar action of the mercury 

 alluded to, the effect of the rays would be multiplied by the number 

 of times the screen was interposed, provided it were always retained 

 long enough to permit the mercury to cool down. The comparison 

 of an instrument thus acted upon with another similar instrument 

 near it, screened entirely from the source of heat, might furnish us 

 with a means of exactly estimating the heating effect of the source. 



The author desires to express his obligation to Mr. Welsh, who, 

 besides finding a use for the instrument, suggested the selection of 

 tubes which appears to answer best, and whose experience was of 

 great assistance in arranging details. He is also indebted to the 

 Kew Committee of the British Association, who kindly examined 

 the instrument, and authorized the construction of several by way of 

 trial. Mr. Casella undertook the operative part in their construction, 

 and his glass-blower, Mr. J. E. Griffin, took pains to discover some 

 of the circumstances that interfere with the proper action of the in- 

 strument, and constructed those that have proved successful. 



Without attempting to explain all the peculiarities of this action, 

 it would seem that the mercury is kept in the narrow bore, and pre- 

 vented from retreating into the bulb, by friction ; but, when a moving 

 force is supplied by means of a change of temperature, the motion of 

 the mercury takes place in that direction in which it is least opposed, 

 or most aided, by its capillary action. 



As the result obtained is due to the difference between two forces, 

 neither of which is very great, the construction of such an instru- 

 ment requires care ; and the author will now state what appear to 

 be the chief points which demand attention, as derived from his own 

 experience, and that of those who have interested themselves in the 

 construction of the tubes ; although this experience is necessarily 

 very limited. 



1st. The tubes should be quite clean and free from moisture. 



2nd. They should be in one straight line, and should expand 

 symmetrically into the bulb. 



3rd. It seems the best arrangement, to have the narrow tube of 

 flat bore, not too flat ; its greatest width being about equal to the 



