18G5.] On the Expansion of Water and Mercury. 551 



Dent, and used by him for finding the longitudes of several observatories 

 in this country. 



On testing 100 chronometers in succession as they passed through the 

 Observatory, the average alteration of daily rate caused by changing the 

 temperature from 40 to 60 was 7 S> ; and in ten per cent, of the hundred 

 the average change was 30 S< 6. 



The chronometer-room at the new Observatory now being erected at 

 Bidston by the Mersey Docks and Harbour Board will be provided with 

 the means of testing simultaneously between two and three hundred chro- 

 nometers in the way shown by the examples in Table I. It is not neces- 

 sary to test chronometers in this elaborate way on every occasion that they 

 arrive in port, as the corrections for change of temperature remain the 

 same fora long period. The rate may change, as in example 2, Table IV., 

 while the thermal correction remains sensibly the same. 



When the Greenwich mean time is communicated from an authorized 

 establishment, as is now generally the case in our large sea-ports, the rates 

 of chronometers in the temperature that prevails at the time can be easily 

 ascertained. At present these rates are used on the assumption that the 

 thermal adjustments are perfect. The corrections for change of tempera- 

 ture in Table II. show the improvement which might be effected by testing 

 all chronometers when new, and supplying mariners with Tables of such 

 corrections as may be found to exist. These corrections would require 

 verifying periodically, as in cleaning and repairing timekeepers the thermal 

 adjustment is sometimes altered. 



December 21, 1865. 



Sir HENRY HOLLAND, Bart., Vice-President, in the Chair. 

 The following communications were read : . 



I. " On the Expansion of Water and Mercury." By A. MATTIIIESSEN, 

 F.R.S. Received December 7, 1865. 



(Abstract.) 



Before commencing a research into the expansion of the metals and their 

 alloys, it was necessary to prove that the method I intended to employ, 

 namely that of weighing the metal or alloy in water at different tempera- 

 tures, would yield good and reliable results. 



To check, therefore, the method, I was led to determine the coefficient 

 of expansion of mercury, and, basing my calculations on Kopp's coefficients 

 of expansion of water, I expected to obtain Regnault's coefficient of expansion 

 of mercury. The coefficient deduced from experiments did not agree with 

 Regnault's ; and being unable to discover any source of error in the method 

 of experimenting, I determined to reinvestigate the matter. 



The memoir is divided into four parts. 



