132 Apparatus for freezing Water hy the aid of Sulphuric Acid. 



100 ^ ,,. , . 



chord of 1° (the variation) will be -^^- = 6.25 feet, (this being near 



enough for practical purposes, — or the arc may be used in place of 



its chord.) From the end of the curve measure back or forward 



6.25 feet, in the line of its tangent, and from that point set off to the 



6.25 

 right or left the variation for —^, which is 0.014 ieet, and the point 



thus found will be the end of the curve, from which the tangent be- 

 ing produced will nearly coincide with the grading line. The va- 

 riation is found by the following proportion. 

 100 feet : 0.44 (the variation in 100 feet) : : ^ 

 6.25 



The annexed diagram will serve as an illus- 

 tration. 



Here by producing the curve from A to C, 

 the direction of the line is changed from AB 

 to CD. The 6.25 was measured on the line 

 AB, as before stated, for the difference be- 

 tween the line AB and the arc AC, or its 

 chord, is so inconsiderable that either may be 

 used, in this case, with equal accuracy in 

 practice. 



Art. XIV. — Apparatus for freezing Water by the aid of Sulphuric 

 Acid; by R. Hare, M. D. Prof, of Chem. in the Univ. of Penn. 



The congelation of water by its own vaporization, accelerated by 

 exposure to the absorbing power of sulphuric acid, or other agents, 

 in vacuo, has always been a difficult experiment. A distinguished 

 professor complained to me lately of want of success in his efforts to 

 repeat it. In November, 1832, after having three times succeeded 

 in freezing water by the process in question, yet having failed before 

 my class, I was led to give more than usual attention to the process 

 in order to obviate the causes of disappointment. It appeared to me 

 that the failure arose from imperfection in the vacuum. An excel- 

 lent pump, with perfectly air tight cocks, is indispensable ; and not 

 only must the pump be well made, it must likewise be in good order. 

 Neither should the packing of the pistons, the valves, nor the cocks, 



