314- On a new circulai- Microincier. 



up with the cork exactly as it "went doxm, but the bottle was full 

 ot salt-iioater. I never examined the watei' with sufficient ac- 

 curacy to be able to say whether it was of the precise saltness 

 of the water in which it had been immersed ; but from my 

 personal observation I am certain it was not by any means the 

 water which was sent down ; the water brought up was turbid, 

 smelt strongly, and had a nauseous saline taste like the sea- 

 water. 



" On various occasions, upon examining the coi'k, it was 

 found that the "water had not eveti penetrated to it, so well had 

 it been secured ; and upon drawing it, and cutting it across, it 

 'was found perfectly dnj. 



" The bottles used were porter ones ; they were generally 

 kept down for a short time, not exceeding a quarter of an hoiu", 



" These experiments were performed on board the Minerva 

 fi'igate. I was twice on deck during the whole of the process 

 of the experiments: the first time was in June 1811, about the 

 latitude of 40° North; the second time was in May 1812, 

 near the northern tropic. I remain yours respectfully, 



" Mr. John Deuchar, 4'c" " John Geant. 



I have laid before the Societj', with the greater satisfaction, 

 the account given by Mr. Grant, as I find it in most respects 

 established by the facts since mentioned by Mr. Campbell in 

 his account of his travels in South Africa, and still more re- 

 cently by those introduced by Mr. Perkins into his paper upon 

 the compressibility of water. 



[To be continued.] 



LVII. On a new circidar Micrometer : communicated in a Let- 

 ter from M. LiTTROW to F. Baily, Esq. 



JIave you heard of the suspended circidar micrometer made 

 by Frauenhofer at Munich ? You are aware that this simple 

 instrument is in general use on the continent, particularly 

 amongst the German astronomers. If used in the manner I 

 have described in my Astronomic (vol. i. page 358), it is cer- 

 tainly a ve7-y useful instrument : and M. Delambre is wrong in 

 treating it so contemptuously in his work on astronomy. How- 

 ever, it has hitherto been difficult to obtain a perfect circle, 

 which should be unconnected with the sides of the telescope : 

 for, if it had been ever so well finished, it was easily bent by 

 taking it off its support, and fixing it in the tube. And it may 

 perhaps be doubted whether there has hitherto been one perfect 

 micrometer, unices the diaphragm itself were used for that 



puipose : 



