106 POPULAR LECTURES AND ADDRESSES. 



cent, of its present value ; he should take a vibrator 

 better arranged for permanence and for absolute 

 accuracy, than the balance-wheel with its hair- 

 spring of a watch or a chronometer. A steel 

 tuning-fork, which has had its period of vibration 

 determined for him, before he leaves the earth, 

 by Professor Macleod, or by Lord Rayleigh, will 

 serve his purpose. By measuring the period in 

 terms of mean solar seconds, with the prongs up, 

 and horizontal, and vertically down, he will be 

 able to eliminate the slight effect of terrestrial 

 gravity ; and he will have with him a time-standard 

 that will give him the mean solar second, as 

 accurately as his measuring-rod gives him the 

 centimetre, in whatever part of the universe, and 

 at whatever time, now or millions of years later, 

 he has occasion to use his instruments. 



I hope that you will not feel that I am abusing 

 your good nature with an elaborate frivolity, when 

 I ask you to think a little more of the unital 

 equipment of our ideal traveller, on a scientific 

 tour through the universe. For myself, what 

 seems the shortest and surest way to reach the 



