67 
“ earth currents,” in a submerged conductor, with telegraphic 
signals, might be obviated by employing a return wire instead 
of the ocean in order to complete the circuit. 
A Paper was read by Dr. Joule, entitled “ Note on 
Dalton’s Determination of the Expansion of Air by Heat.” 
Regnault, following Rudberg and Gilbert, had attributed to 
Dalton an error in reducing his experiments on the expansion 
of air between 55° and 212° Fahrenheit to that from 32° to 
212°. The Author showed, by reference to Dalton’s pub- 
lished papers, that his commentators had entirely misunderstood 
the real facts, the truth being that Dalton in stating that his 
coefficient coincided wdth Gay Lussac’s, referred to some 
unpublished experiments which he had made subsequently 
to those described in the Manchester Memoirs. The experi- 
ments of Dalton justified him in drawing the approximately 
accurate conclusion that all elastic fluids under the same 
pressure expand equally by heat ; the importance of which 
law to the theory of heat was at once obvious to his sagacious 
mind. 
In the conversation which ensued. Professor Calvert 
pointed out the necessity of employing perfectly pure mer- 
cury in the construction of thermometers. The presence of 
a very minute quantity of tin would alter its conducting 
power very considerably, and probably the uniformity of its 
rate of expansion. The Rev. H. H. Jones and Mr. Atkinson 
made some remarks as to the height of the earth’s atmosphere, 
and whether the cause of variation of its pressure arose from 
waves similar to tidal waves. Mr. Atkinson considered that 
the greatest variations of pressure were near the surface, 
while Professor Christie and Dr. Joule, from the phe- 
nomena observed on high mountains, and in balloons, were 
of opinion that the velocity of winds is greatest at high 
elevations. 
