of Edinburgh , Session 1879 - 80 . 
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abundance of ice and salt to make freezing mixtures ; and with no 
other apparatus than can he made by a moderately skilled glass- 
blower ; and with no other standard of physical measurement of any 
kind than an accurate linear measure. He may assume the force of 
gravity to be that calculated for his latitude with the ordinary 
rough allowance for his elevation above the sea, and his omission to 
measure with higher accuracy the actual force of gravity in his 
locality can lead him into no thermometric error which is not in- 
comparably less than the inevitable errors in the reproduction and 
use of the air thermometer, or of mercury or other liquid thermo- 
meters. In temperatures above the highest for which mercury- 
steam pressure is not too great to be practically available, nothing 
hitherto invented but Deville’s air thermometer with hard porcelain 
bulb suited to resist the high temperature is available for accurate 
thermometry. 
The following statement is in the Encyclopaedia Britannica 
article “ Heat,” appended to the description of steam-pressure thermo- 
meters which it contains : — “ We have given the steam thermo- 
meter as our first example of thermodynamic thermometry because 
intelligence in thermodynamics has been hitherto much retarded, 
and the student unnecessarily perplexed, and a mere quicksand has 
been given as a foundation for thermometry, by building from the 
beginning on an ideal substance called perfect gas, with none of its 
properties realised rigorously by any real substance, and with some 
of them unknown, and utterly unassignable, even by guess. But 
after having been moved by this reason to give the steam-pressure 
thermometer as our first theoretical example, we have been led into 
the preceding carefully detailed examination of its practical qualities, 
and we have thus become convinced that though hitherto used in 
scientific investigations only for fixing the “boiling-point,” and 
(through an inevitable natural selection) by practical engineers for 
knowing the temperatures of their boilers by the pressures indicated 
by the Bourdon’s gauge, it is destined to be of great service both in 
the strictest scientific thermometry, and as a practical thermometer 
for a great variety of useful applications.” 
