THE MANOMETERS. 29 



form bore throughout, and that the form of the bore in every part shall 

 be circular. Very few, if any, tubes conform perfectly to both require- 

 ments. The material from which selections are to be made is imported 

 in lots of several kilograms each, and the purveyors are urged to spare 

 neither pains nor expense in procuring tubes of the highest possible 

 excellence. In each lot of selected material thus obtained, there are 

 usually found — though not always — a few tubes which answer all rea- 

 sonable requirements. 



The first step in making a manometer is to etch upon the tube two 

 fine lines extending completely around the instrument. These are 

 usually referred to as the "upper scratch" and "lower scratch," one 

 being near the upper and the other near the lower limit of the calibrated 

 portion of the manometer. These lines are made no coarser than is 

 absolutely necessary in order that they may be distinctly seen through 

 the telescope, since in small tubes a meniscus behind any line, however 

 fine, is apt to give the observer trouble. No other graduation appears 

 upon the manometers. All readings on the instruments are referred to 

 one or the other of the two "scratches." That is, a reading consists 

 always in determining the distance between the meniscus of a mercury 

 column and either one of the lines in question. Since the distance 

 between them is accurately known, readings referred to one line can 

 readily be transferred to the other. The distance between the lines 

 depends upon the length which the manometer is to have — ultimately, 

 of course, upon the height of the available space in the baths. Above 

 the upper and below the lower scratch, a considerable length of tube 

 is left to provide for subsequent operations. 



Two methods of calibration have been employed, both of which will 

 be briefly explained, though the earlier one is not now much in use 

 except for preliminary explorations of the tubes. 



FIRST METHOD. 



Figure 16 represents the instrument which is employed to move and 

 adjust the calibrating thread. A steel screw (a), with a long lever, is 

 threaded through a cap of hard rubber (b), in which the glass tube (d) — 

 enlarged at c — is set with litharge-glycerine cement. In order to make a 

 mercury-tight joint, the upper end of the steel screw is slightly lubri- 

 cated, and around the portion which extends into the glass tube some of 

 the cement is allowed to solidify. The rubber stopper (/), carrying the 

 manometer (g), is inserted (with the aid of the "fang," Figure 8) in the 

 glass tube (d), which is sharply contracted at the upper end (e). 



The manometer is drawn out at the upper end to a fine tube which is 

 bent into the form of an inverted U . With the apparatus — including 

 the manometer — nearly full of mercury, the screw is turned to the right 

 until the column enters and reaches the highest part of the inverted U . 



