271 



If a convenient minimum thermometer could be constructed, the 

 effects of very great pressures might easily be tested by hermetically 

 sealincr the thermometer in a strong glass, or in a metal tube, and 

 putting it into a mixture of ice and water, in a strong metal vessel, 

 in which an enormous pressure might be produced by the forcing 

 pump of a Bramah's press. 



In conclusion, it may be remarked, that the same theory which 

 pointed out the remarkable effect of pressure on the freezing-point of 

 water, now established by experiment, indicates that a corresponding 

 effect may be expected for all liquids which expand in freezing; that a 

 reverse effect, or an elevation of the freezing-point by an increase of 

 pressure, may be expected for all liquids which contract in freezing ; 

 and that the extent of the effect to bo expected may, in every case, 

 be deduced from Regnault's observations on vapour (provided that 

 the freezing-point is within the temperature-limits of his observa- 

 tions), if the latent heat of a cubic foot of the liquid, and the altera- 

 tion of its volume in freezing be known. 



5. On the Extinction of Light in the Atmosphere. By W. 

 S. Jacob, Esq., H.E.I.C. Astronomer, Madras. Com- 

 municated by Prof. C. Piazzi Smyth. 



In a letter dated Madras, November 1 849, Captain Jacob says, 

 " I have been much interested in reading, lately. Professor Forbes's 

 paper in the Philosophical Transactions, 1842, Part 2, on the Ex- 

 tinction of Light and Heat in the Atmosphere." As his results 

 agree very closely with those of my experience on the Trigonome- 

 trical Survey of India, and which, though not founded on any pre- 

 cise measures, being still the conclusions of some years' experience, 

 are perhaps worth noticing, particularly when they agree with the 

 results of more exact measures. 



On commencing work with heliotropes in 1837, I soon found that 

 for long distances it was necessai'y to enlarge the apertures more 

 than in the simple ratio of the distance (though such was Colonel 

 Everest's practice) ; and before the end of the first season, I had 

 formed a scale of apertures for corresponding distances, which aftei'- 

 wards needed very little alteration, but when finally corrected by 

 subsequent years' observation, stood as follows : — 



