54 Notes on Barometer Construction. 



" The bend of this instrument it is advisable to form of 

 tube of very small diameter; in which case, even with 

 awkward carriage of the filled instrument in the labora- 

 tory, and even when it is violently shaken, air cannot pass 

 from the open to the closed limb. 



"During numerous comparisons of this instrument with 

 an excellent standard barometer, of unusually large cross 

 section, it yielded excellent results. 



" The first instrument made by the author, rather faulty 

 in the dividing and in the grinding in of the stop-cock A, 

 he has sent to the Kensington Exhibition of Scientific 

 Instruments. It had a not very suitable iron stand. 



" Aschafienburg, 25th July, 1876." 



Guthrie's proposition aims at increased sensitiveness in the 

 reading. In the first place he makes mention of a propo- 

 sition long lost sight of and due to Descartes, in which is 

 employed a column of dense fluid mercury ; but in conjunc- 

 tion with a super-posed column of a much less dense fluid, 

 in terms of which latter the atmospheric pressure is measured. 

 Descartes' proposition included an aqueous solution of tartar 

 emetic above the mercury; the object of employing this 

 fluid solution being that of ensuring the expulsion of air 

 Mr. Guthrie proposes to substitute glycerine or heavy hydro- 

 carbon oil instead of the tartar emetic solution. Guthrie 

 states, in reference to the diagram (Fig. 3) which he gives 

 of this form of barometer, that '' the sensibility of such a 

 barometer would obviously be, if the upper liquid were 

 without weight, directly proportional to the ratio between 

 the sectional areas of the cylindrical chamber and the upper 

 tube (if also the open limb were of infinite area). But the 

 upper liquid having weight, the limit of sensibility is the 

 comparative density of mercury and the liquid (say 16: 1) ;* 

 accordingly this limit is secured when the cylindrical 

 chamber has four times the diameter of the upper tube." 



Professor Guthrie adds his own suggestion of a syphon 

 barometer with a horizontal capillary tube of relatively 

 great length connecting the column and cistern, the 

 measurements being made on the capillary tube, in which 

 a small bubble of air or fluid is intercalated dividing the 

 mercurial cylinder (Fig. 4). Without doubt the indications 

 of change of such an instrument are very sensitive ; indeed, 



* Hypothetical gravities, for simplicity of illustration. 



