344 PHILOSOPHICAL TRANSACTIONS. [aNNO 1686. 



Upon this, in June 1668, he bethought himself of another device, which 

 was to increase the divisions, by putting coloured spirit of wine, or some other 

 liquor not liable to freeze on the mercury, which liquor was made to rise as the 

 mercury fell, and fall as it rose, in a narrow tube, so as to make the utmost 

 limits about two feet asunder. Mr. Hook, however, was not yet satisfied, till 

 he had found out the means of increasing the divisions of the barometer at 

 pleasure, which was done in the following manner. 



Fig. 12, pi. 9, represents the glass of this baroscope: the cylinder A maybe 

 of any diameter you please, the larger the better ; but it need not be above 1 

 inches long. The tube AB must be so long, that the upper part of the cylinder 

 B may be 29 inches + such a part of the height of the other tube B C, as the 

 weight or specific gravity of the liquor which is to fill that tube, is to the specific 

 gravity of mercury, below the line ab in the cylinder A. The third cylinder C 

 may be as high as you please above the cylinder B, but is most conveniently 

 made so that the square of the diameter of the tube B C, be to the square of 

 the diameter of the cylinder's B or C, which must be exactly equal, as the rise 

 of the mercury in the cylinder B, is to the whole length of the tube B C : for 

 in this case there will be nothing superfluous, but the divisions enlarged to the 

 utmost advantage. 



One way of filling this barometer, is to leave a small hole at the top of the 

 cylinder A, and another near the top of the cylinder B: this latter being well 

 stopped, pour in as much mercury at the other hole in A, as shall fill both 

 tubes as high as the level of the said hole ; which done, stop, either by her- 

 metically sealing it, or else by a drop of sealing-wax, the hole A ; then opening 

 the hole in B, draw off as much of the mercury of the tube B C, till it will 

 run no longer; which done, stop firmly the hole in B, and you will have the 

 cylinder A evacuated of air; and the height of the mercury will be as is usual 

 in the ordinary plain and wheel-barometers. Then pour into the tube B C as 

 much spirit of wine tinged with cochineal and oil of turpentine, equal parts of 

 each, as shall stand above the surface of the mercury, so many feet as you make 

 the enlarged scale of your barometer, or as is between the middle of the 

 cylinders B and C; and you will find the mercury sink in the tube B C, and rise 

 in the other tube AB, in such proportion, that each 13 feet of oil and spirit 

 will raise the mercury 10 inches: this done, you must pour on, by the tube 

 B C, as much mercury as may fill up the cylinders A and B to such heights, 

 that the surface of the mercury in both, may, at the utmost limits, (which 

 have not in England been found to exceed 30.6 and 28.6 inches) always fall 

 within the bodies of the cylinders, and never enter into the tubes. 



The effect of this baroscope will be, that when the atmosphere is heavy, and 



