Registering and Printing Barometer. 15 



could be obtained without interposing any substance ; and 

 these would undoubtedly be the best, as less liable to in- 

 jury from accident. 



As before remarked, we have adopted the siphon form 

 of barometer ; but we are not limited exclusively to this : 

 since an ordinary cistern barometer could have been used 

 with equal facility, by simply inverting it and bending the 

 lower end in a siphon form. With this we would get nearly 

 double the motion which we now obtain, for the same 

 amount of pressure; but there would be a variable correc- 

 tion which it would be necessary to apply to every record, 

 for eliminating the error due to capacity of cistern. By 

 making the cistern large in diameter and the siphon small, 

 this correction might be reduced to a small quantity ; yet 

 where we desire accurate results, it would nevertheless be 

 necessary to apply it. 



Taking all the advantages and disadvantages in consid- 

 eration, we decided in favor of the siphon proper; so that 

 now our results are printed in decimal parts of an inch, 

 with no correction of any kind to apply. 



The screw S, on which the accuracy of the results will 

 in a great measure depend, is, as before remarked, 50 

 threads to the inch, and was cut by Mr. Charles Fasoldt ; 

 and it is believed to be a very perfect one. We have as 

 yet not had a sufficient number of comparisons to deter- 

 mine the eccentricity or irregularity, but it is presumed 

 that it will not be appreciable on the results. 



The best form and arrangement of the float can only be 

 determined by experiment. In our original barometer 

 tube, which was 0.4 in. diameter, we inserted an ivory float 

 which nearly fitted the tube. The upper end of the wire 

 for supporting the platinum disk d, was connected with 

 the arm carrying the platinum points. This form worked 

 very well, as the following results will show. The instru- 

 ment was allowed to run from April 24th to 30th, without 



