10 



Mr. J. P. Gassiot on Observations [June 20, 



at Yarious places during tlie voyage. In order to make use of these 

 records, it became necessary to ascertain tlie corrections of tlie instru- 

 ment. 



Immediately on receipt of Captain Mayne's letters, I forwarded 

 them to Mr. Stewart, to whom I am indebted for the following obser- 

 vations, with the necessary corrections for temperature, &c. These 

 corrections are, — 



I. The Temperature Correction. 



In order to determine the correction, very complete sets of experi- 

 ments were made by Mr. Balfour Stewart at Kew Observatory, and by 

 Mr. Browning at the Minor ies. 



In January 1866 the spectroscope was conveyed to Kew Observa- 

 tory, and was there exposed to a change of temperature equal to 30° 

 !Fahr. The change was applied very gradually, the experiments for one 

 set lasting nearly one month ; in respect to duration these changes 

 were consequently analogous to those to which the instrument would 

 be exposed at sea, but, on the other hand, the instrument, when at 

 Kew, was not subjected to vibrations. 



In February 1865 the spectroscope had been subjected to similar 

 changes of temperature at Mr. Browning's house of business in the 

 Minories, which abuts on the Blackwall railway. The results there 

 obtained are recorded in the Proceedings of the Eoyal Society (vol. xiv. 

 June 1865). These observations extend over a range of 30° Pahr. ; and 

 as in the greater portion of the time during the observations the in- 

 strument was subject to constant vibration from the ordinary work 

 carried on in the work-rooms, as well as from the abutting railroad, 

 these constant vibrations were so far analogous to the action to which 

 the instrument would be subjected to on board a vessel. 



On the other hand, the heating and cooling each took place on the 

 same day, and as far therefore as duration is concerned, the tempera- 

 ture changes at the Minories were dissimilar to those to which the 

 instrument was exposed at sea. 



The result of these experiments was that, for an increase of tem- 

 perature of 30° Pahr., there was observed an increase in the reading of 

 i-32 revolution of the micrometer screw. 



The following Table exhibits the result of the observations made at 

 Kew Observatory: — 



