On an Instrument for registering Changes of Temperature. 453 



ingly a bulb was blown between two thermometric tubes of differently- 

 sized bores, in sucb a manner that the tubes lay in one straight line, 

 with the bulb between them. The bulb was then filled with mer- 

 cury, and the tubes were hermetically sealed at both ends, havmg 

 been first carefully deprived of air. When the instrument thus con- 

 structed was laid in a horizontal position, or nearly so, its action was 

 precisely what the author had hoped ; the mercury contracting from 

 cold only in the narrow bore, and expanding from heat only in the 

 wide one, — even when viewed by a microscope of considerable mag- 

 nifying power. 



It was suggested by Mr. Welsh, Director of the Kew Observatory, 

 that such an instrument might be used for measuring fluctuations of 

 atmospheric temperature ; and the following use afterwards occurred 

 to the author. Were it required to exactly estimate the radiating 

 effect of a source of heat, it might perhaps be 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, M. J. E. Grifiiu, 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 Ime, and should expand 

 symmetrically into the bulb. 



