650 
MR. AIRY’S ACCOUNT OF THE COXSTRECTIOX OF 
and are besides to be converted into pendulums, each oscillating on a knife-edge placed 
, where it makes the time of vibration a minimum. If the period of vibration of the 
same bar at the same place is unchanged by time, gra-vity being supposed unchanged, 
then the whole length of the bar is unchanged ; and the distance of the divisions marked 
on it, supposing the bar to continue straight, is unchanged. The bar must of course be 
guaranteed from rust or change of shape. As the test of actual length is required to 
be very exact, the series of experiments should be continued without interruption as 
long as possible. I have therefore had an apparatus constructed for swinging these 
pendulums in vacuo, which, so far as I can judge, will give almost any degree of accuracy 
to this part of the inquiry, with very little trouble to the observer ; this was in fact my 
first step, and until this was secured I did not think it worth while to apply myself to 
the rest of the apparatus. 
“ The next point is, the very accurate comparison of two bars with each other, the 
bars being supposed of the same material. Connected wth this is, the determination of 
the law of expansion of the metal that may be used, at usual and moderate temperatiu’es. 
The preparations for this apparently simple operation ^vill perhaps appear complicated. 
I should however be unwalling to give up any of them ; so important do I hold this 
solidity of the apparatus to be. 
“ The real difficulties in the practical problem have been, so far as I can judge, the 
instability of the optical beam compass employed, the unsatisfactory mode of supporting 
the bar, the Avant of due verification of the axes of the microscopes, and the variations 
of temperature, Avhich were not indicated by the measuring means employed. Thus, in 
Mr. Baily’s apparatus [the apparatus of the Royal Astronomical Society], which was 
certainly an improvement on all that preceded it, the obseiwer’s body had a sensible 
effect on the bar which carried the microscopes, and probably on the direction of their 
axes ; it also affects differently the temperature of the two bars and that of the thenno- 
meters. These effects may be partly eliminated by interchanging the bars, &c. ; but 
there is another cause of error which cannot be eliminated, namely, that the beam which 
carries the microscopes is affected by the motion of the frame which carries the bars. 
[Mr. Sheepshanks then described an experiment of the comparisons of tliree ii'oii bars, 
in which every single reading of the microscopes Avas very satisfactory, yet the result as 
relating to the difference of length of the three bars Avas misatisfactory.] 
“ Having thus arrived at the conviction of the absolute necessity of a steady stand and 
an immoveable beam compass, I have tried to obtain them in the manner Avhich you 
Avill see. 
“ The principle I have had constantly in my mind has been this. We do not knoAv 
where the difficulties may be : let us not throAV aAvay a chance ; let us take care to pro- 
vide for all knoAvn contingences, regardless of cost, and trust to combating the imknoAATi 
difficulties when they are found. It seemed to me not suitable to the Committee, nor 
indeed to our national character, to try any but the best conceiAable means, and I have 
acted for the Government as I would do for myself. 
