RESEARCHES IN THERMOMETRY. 579 
TABLE VIII, 
a Y455 Found. | Y45, Cale. a yz Found. | y3 Cale. fi. ye Found. | y- Cale. 
0:00 —_ 2°726 0 — 4-000 0:00 — 1:000 
1:05 2°695 2°673 1 3°912 | 3°886 1°35 968 951 
2°61 2°567 2558 2 3°750 | 3°745 2°76 ‘872 878 
3°12 2°353 2°509 3 3567 | 3°570 3°87 "795 “799 
4:05 2°209 2°412 4 3340 | 3°356 5:03 688 686 
5:02 2°264 2°256 
6:00 2092 2°065 
6°71 1-840 1891 
7-67 1590 1584 
8:28 1:397 1:365 
9°15 1:088 ‘944 
9°86 616 ‘554 
Probable error of a 
single compari- 
sion . , oh 0°:057 Be 4a 0°:003 ae ane 0°:007 
The starting-point of the observations with thermometer 455 was 168°'6. 
When in its equation y=0, x=10°61 ; the ascent under the influence of heat is 
thus converted into a depression after 306°°5. Similarly, the starting-point for 
thermometer ¢ was 48°°1. The value of x corresponding to y=0 is in this case 
8'728 ; the ascent is therefore converted into a depression after 379°'8. On the 
other hand, under the influence of a lower range of temperature commencing 
at 40°, the zero of thermometer 3 descends until z=10°828, corresponding to 
256°'7 C, after which it would begin to rise. 
It is evident from these experiments that the zero of an ordinary vacuous mer- 
curial thermometer undergoes three distinct movements under the influence of 
heat. It is at first depressed by the action of temperature varying with each par- 
ticular case, the mean of the numbers actually obtained being at the upper limit 
(48°-+ 143°5 + 168°°6 + 256:7) _ 
; =1542. The further application of heat causes the 
= 343°-2, 
zero to ascend, the effect in this direction being limited to cess) 
after which it again descends. The explanation of these phenomena is probably 
as follows. The first effect of heat on the bulb is ordinary expansion, attended 
with a “set.” The second or further effect is to cause the thin part of the glass 
to become sufficiently plastic to yield to the influence of barometric pressure, 
which causes gradual collapse. In the third or final stage, at which the vapour 
of mercury has a sensible tension, the bulb is enlarged by outward pressure 
due to that cause, and the zero descends. 
If the above view be correct, it would follow that a thermometer open to 
