29(3 Mr. S. W. Holman on a Simple Method 



In the method which will now be given, either one or both 

 of these points may be left to be selected, according to the 

 combined conditions of length of thread employed, shape of 

 the tube, and numerical convenience, after the observations 

 with the thread have been made. 



Let it be desired to find the calibration-corrections for a 

 given tube. Determinations which will give the errors of 

 every 3 centim. of length will ordinarily be sufficient ; but this 

 must depend on the result sought. Separate a thread of mer- 

 cury of about that length. The actual length of the thread 

 within two or three millimetres is of no consequence whatever; 

 and hence a suitable thread can be obtained in a very short 

 time. 



Set the thread with its lower end at or near the beginning 

 of the graduation : call the reading* of the lower end of the 

 thread l 1} and that of the upper end u v Move the thread less 

 than 1 millim. and read again, finding thus l 2 and v 2 . Move the 

 thread about 1 centim., and read l s and u z . Move the thread less 

 than 1 millim., and read ^ and w 4 . So continue throughout the 

 whole length of graduation, increasing the number of settings 

 or repeating the whole series in reverse order and several 

 times, if the highest attainable precision is desired. This 

 alternation between 1 millim. and 1 centim. in setting tends 

 towards the better elimination of errors in estimation. It is 

 not, however, essential, nor even always as well as an equal 

 number of distributed readings. This must depend upon the 

 skill of the observer. Avoid, as far as convenient, taking 

 readings with an end of the thread apparently just at the line 

 of the scale, as the width of the line, even in the best scales, 

 is a source of considerable errorf . If any point (e. g. the 

 zero-point of the graduation) has for any reason been selected 

 as the first of which the error should be assumed zero, the 

 settings may to advantage, though not necessarily, be made 

 to extend each way from this. 



Then ii\ — l\, u 2 — I 2 , &c. will give a series of lengths of the 

 calibrating-thread in all parts of the tube. Before reuniting 

 this thread to the rest of the mercury, plot points with abscissas 

 Zj, 1 2 , &c, and ordinates ii x — l u n 2 — I 2 , &c, the corresponding 

 lengths of thread, and draw a smooth curve through the 

 points thus obtained. This line will give a general idea of the 

 form of the capillary bore ; and should any parts of it show 

 considerable irregularities, the corresponding portions of the 

 tube should at once be reexplored with the thread. 



If not already done, the point A upon the scale, to be used 



* Tenths of a division are supposed to be read by estimation. 



t Some of the advantages of Neumann's method are offset bv this error. 



