of Edinhunjh, Session 1883 - 84 . 
577 
Principle of Chronometry. 
From the foregoing law it is readily deducible, as a corollary by 
elementary matbeinatical considerations, that — 
Any dial-traveller which would accomplish the conditions stated 
would make progress proportionally with any other dial-traveller, 
obtained likewise from the same set of bodies, or any other set of 
bodies with the same or any other reference frame. Then, in view 
of this remarkable agreement, we define as being equal intervals of 
time, or we assume as being somehow in their own nature intrinsi- 
cally and necessarily equal intervals of time, the intervals during 
which any such dial-traveller passes over equal spaces on its dial. 
Thus, any dial-traveller which would accomplish the conditions 
stated would constitute a perfect chronometer. 
This gives us the ideal of a perfect chronometer. It remains for 
men to aim at ap23roaching as near as they can towards that ideal 
in the practical realisation of good chronometry. 
For good and long-enduring realisations of chronometry, astro- 
nomical methods are alone available, l^one of these present any 
simple method of procedure. They require hypothetical assump- 
tions of supposed forces acting on the bodies considered, and, above 
all, there is involved in them the assumption, and after multitudinous 
tests, accompanied by multitudinous confirmations, the discovery 
of the Law of Universal Gravitational Attraction — the grandest of 
the discoveries of Sir Isaac Uewton. 
Principle of Absolute Clinural Rest and of Absolute Rotation. 
Any straight line fixed relatively to any reference frame which 
accomplishes the conditions specified in the statement of the law of 
inertia has absolute clinural rest. If another straight line fixed in 
any other such reference frame be parallel to that former line, the 
two lines will continue parallel, so that by either of them the one 
same absolute clinure is permanently preserved. The principle 
here called that of absolute clinural rest is clearly enunciated in 
Thomson and Tait’s Natural Philosophy, § 249, under the designa- 
tion of “ Directional Fixedness.” It is there exhibited by a very 
simple device, and here by a somewhat different method. 
