132 
MR. J. R. CHRISTIE ON THE BAROMETRIC THERMOMETER. 
of its removal from place to place under difficult circumstances, is not exclusively to 
be measured by the degree of delicacy or even of accuracy of its indications ; since 
however perfect it may be theoretically, when stationary, it can never be of practical 
benefit unless it be of such a construction as to bear the concussions and shaking it 
must be necessarily exposed to when conveyed in the manner in which it can alone 
arrive at the point where its agency is required. This construction has been attained 
in the instruments by means of which are determined the two principal coordinates 
of geographical position, under all circumstances involving difficulty of access to 
the place of observation, while the third, height above the sea level , has had almost 
exclusively applied to its determination an instrument more liable than almost any 
other to suffer daring removal, and, from its very nature, incapable of any great im- 
provement in this respect. With regard to the instrument under consideration, I 
can, in reference to the above points, speak most positively; not only were the con- 
cussions which it constantly suffered very considerable, but on two or three occa- 
sions it escaped uninjured by blows which must have shivered any common barometer, 
however externally protected ; and it has only at length been destroyed through 
having been inadvertently left boiling; — the water having evaporated, the increase of 
temperature of the empty boiler proved too great for the capacity of the tube of the 
thermometer, and the bulb burst. 
Since writing the above my attention has been directed to the researches of 
Holtzmann and Magnus, given in Vol. iv. Part 14. of the “ Scientific Memoirs,” and 
I have applied the formula for the expansive force of steam deduced by the former, to 
the determination of an expression for the difference of level in terms of the difference 
of boiling temperature. Reduced to English measures, Holtzmann’s result gives 
h 39 0 — 4 25'2 X Io g/3 + '7 488 g HI. 
b 61 —4 Id IX 6-73157-log /3 
b and /3 being the same as in (I.). 
Applying this as before, in the case of De Luc’s, I obtain 
191939000 
H = 
r. (b-b'){l + (t— 32°)-00222} 
IV. 
(393-2 +b) (393-2 + b 1 )' 
The results derived from (III.), within the limits of my observations, differ but little 
from those obtained from (I.), but have a tendency to diminish the boiling tempe- 
rature more rapidly as the pressure decreases, as the following results sufficiently 
show : — 
Pressure. 
72174 mm. 
561*61 mm. 
Temperature. 
De Luc. Holtzmann. 
209-386 209-425 
Diff. 
+ •039 
— •152 
197-389 197*237 
Even were this last difference greater, we must still rely upon De Luc’s until it can 
be shown that the results obtained with (III.) are more nearly in accordance with ob- 
servations made under natural atmospheric pressures than those derived from (I.) 
have been found to be. 
The practical disadvantages of (IV.) are evident. 
