VOL. LXXVIII.] 



PHILOSOPHICAL TRANSACTIONS. 



431 



al ter during their voyage home ; and consequently their strength, at the time the last 



observations were made with them, could not differ much from that here set down. 

 It would be tedious to give the experiments 



for determining their freezing points in detail; 



but from these experiments it should seem, that 



the freezing point of oil of vitriol, answering to 



different strengths, is nearly as annexed : hence 



we may conclude, that oil of vitriol has not only 



strength of easiest freezing, as Mr. Keir has shown ; but that, at a strength 



superior to this, it has another point of contrary flexure, beyond which if the 



strength be increased, the cold necessary to freeze it again begins to diminish. 

 The strength answering to this latter point of contrary flexure must probably 

 be rather more than .9I8, as the decanted or unfrozen part of N° 2 seemed 

 rather stronger than the undecanted part ; and for a like reason the strength of 

 easiest freezing is rather more than .846. Mr. Keir found that oil of vitriol 

 froze, with the least degree of cold, when its specific gravity at 6o° of heat was 

 I.78O, and that the freezing point answering to that degree of strength was -\- 

 46°: which agrees pretty nearly with these experiments, as the strength of oil of 

 vitriol of that specific gravity is .848, that is, nearly the same as that of N° 3. 



j4 Meteorological Journal kept at the Apartments of the Royal Society, by Order 

 of the President and Council, p. 1 9 1 . 

 The result of the whole is as follows : 



1787. 



January . 

 February 

 March . 

 April . . . 

 May . . . 

 June . . . 

 July ... 

 August . 

 September 

 October . . 

 November 

 December 



"Whole y ea 



Explanation of the Instruments. — The instruments with which the foregoing 

 observations were made are the same that were used in former observations of this 



