THE MANOMETERS. 45 



meter on the graduated scale. It consists of a sliding collar (a), on the 

 upper and heavier end of which has been cut a thread of 1 millimeter 

 pitch. Over this runs the internally threaded collar (b); and upon b 

 rests the sleeve (c) on which is mounted the telescope. The collar (6) is 

 graduated in 100 equal parts, while c has engraved upon it a vertical 

 zero line. One entire revolution of b corresponds therefore to a rise or 

 descent of 1 millimeter in the telescope, and its movements up and 

 down can be read directly to hundredths, and estimated to thousandths 

 of a millimeter. The device is a substitute for the usual micrometer 

 eye-piece on the telescope, and has the advantage over the latter that 

 it is not necessary to have a precisely fixed distance between the eye- 

 piece and the graduated scale. A second advantage, considered as a 

 means of elevating and lowering the telescope, is that the whole weight 

 of the telescope and its balanced carriage is uniformly distributed upon 

 the top of the collar (b) and ultimately upon the upper side of the thread. 

 Hence, when the collar is turned, there is neither any of that "lurching" 

 of the telescope which is so offensive in the older arrangements, nor any 

 "back lash" on the thread. 



THE FILLING OF THE MANOMETER. 



When the manometer has been calibrated and the value of the menis- 

 cus correction ascertained, and the extent of the capillary depression has 

 been determined at a great many points, it is joined to the uncalibrated 

 portion of the instrument and filled with nitrogen. 



Originally the manometers were filled with purified and dried air, but 

 it was found that, however pure the mercury in them might be, the 

 volume of the included air slowly diminished. At first it was suspected 

 that this diminution in the volume might be only apparent; in other 

 words, that the capacity of the manometers was increasing under the 

 pressures to which the gas was subjected. To test this suspicion, long 

 columns of mercury were placed in calibrated tubes, like those used for 

 manometers, between columns of air; and these were then subjected to 

 pressures equal to the highest osmotic pressures which were being- 

 measured. The purpose was to discover whether the columns of mer- 

 cury, under such treatment, diminished sensibly in length — either 

 temporarily or permanently. The results were wholly negative. It 

 was therefore concluded that the observed decrease in the volume of 

 the imprisoned air must be due to the action of the oxygen on the mer- 

 cury, though no fouling of the glass, such as would be expected from 

 the presence of oxides, had been noticed. A third possible explanation, 

 namely, that in the course of the movements of the mercury back and 

 forth some of the gas had been "rubbed out" of the tubes, was not seri- 

 ously considered. If the loss in volume of gas was due to the disap- 

 pearance of oxygen, the obvious remedy was to fill the manometer with 

 nitrogen. The remedy was so complete that, after years of use, no 

 change in the volume of that gas in the manometers has been observed. 



