212 
LORD EAYLETOH ON A NFAV MANOMETER, 
In many kinds of observation the zero can only be verified at intervals, as it 
requires the 2 )ressures over the mercury to be equalised. On the whole the zero was 
tolerably constant to withiii two or three-tenths of a millimetre of the scale. A 
delicate level was attached to the telescope to give warning of any displacement of 
the stand (all of metal) or of the ground. 
The differences of pressure to Ije evaluated are not quite in simple proportion to 
the scale reading from zero. The latter varies as tan 20, while the former varies as 
sin 0. Tlie correcting factor is therefore 
1 , = 1 — ^6' apinoximatelv. 
I tan 26 - 11 
If the zero reading (in millimetres) he a, and the current reading x, D the distance 
l)etween telesco})e and mirror, 
6 — ^ .qyp apprf)ximately ; 
so that the correcting factor is 
1 _ 3 
- • 
The actual correction to be applied to [x — a) is thus 
__ 3 (■^ - - 
41R ■ 
in practice {x — a) rarely exceeded 350, for wliich the correction would be — f’6. 
When (.r — a) falls below 120, the correction is insensible. 
The ]iext question is the reduction to absolute measure. What (corrected) scale¬ 
reading corresponds to 1 millim. actual diftereiice of mercury levels ? The distance 
between the points is 27’3 millims., so that 1 millim. mercury corresponds to 
231 millims. of the telescope scale. The highest pressure tliat could be dealt with is 
about 1-^ millims. of mercury. 
The above reckoning proceeds upon the supposition that the distance between the 
points can Ije regarded as invariable. Certain small discrepancies manifested at the 
liigher slopes of the apparatus induced me to examine the question more jDarticularlv, 
for it seemed not impossible that owing to the bending of the glass-work some 
displacement miglit occur. But a rather troublesome measurement of the actual 
distance in various positions l)y means of microscopes negatived tliis idea. I would 
liowever recommend that this })oint be kept specially in view in the design of any 
snbsecpient apparatus of this kind. 
