CHAP, v.] GENERAL CONCLUSIONS. 303 



C. to vary bet^yeen the extremes of 102780 and 1024:00; so 

 that, to be of any vahie at all, the possible error in the results 

 must not exceed 10. The hydrometer used for these observa- 

 tions is fully described in a paper presented to the Royal Soci- 

 ety by Mr. Buchanan early in 1875, and j)ublished in abstract 

 in the " Proceedings " for that year. Its description is briefly 

 as follows : 



The stem, which carries a millimetre-scale 10 centimetres long, 

 has an outside diameter of about 3 millimetres, the external vol- 

 ume of the divided portion being 0'8607 cubic centimetre ; the 

 mean volume of the body is 160*15 cubic centimetres, and the 

 weight of the glass instruinent is 160*0405 grammes. With 

 this volume and weight it floats in distilled water of 16° C, at 

 about the lowest division (100) of the scale. In order to make 

 it serviceable for heavier waters, a small brass table is made to 

 rest on the top of the stem, of such a weight that it depresses 

 the instrument in distilled water of 16° C. to about the topmost 

 division (0) of the scale. By means of a series of six weights, 

 multiples by 1, 2, 3, 4, 5, and 6 of the weight of the table, spe- 

 cific gravities between 1-00000 and 1*03400 can be observed. It 

 is not necessary that these weights should be accurate multiples 

 of the weight of the table ; it is sufficient if they approach it 

 within a centigramme, and their actual weight be known with ac- 

 curacy. The weights of the table and weights in actual use are : 



Weight of table 0-8360 gramme. 



" of weight No. 1 0-8560 " 



" " II 1-6010 



" " III 2-4225 grammes. 



" IV 31245 " 



" " V 4-0710 " 



" " YI 4-8245 



For ocean-waters the hydrometer is always used with the table 

 and either No. lY. or No. V. weight. 



When the mechanical part of the construction of the instru- 

 ment was finished, with the exception of the closing of the top 



