254 TENTH ANNUAL REPORT OP 



pillarity, and thus avoid at once tlie labor of applying a correction 

 and the risk of error from an accidental neglect of it. 



It mnst he home in mind that this correction applies only to the 

 particular tube, and while preserved in good condition. 



If this tube is injured and again used, or another tube put in its 

 place, the scale should then be moved until the lines coincide, the 

 amount of correction for the repaired or the new tube being estimated 

 until a good comparison can be made directly or intermediately with 

 the Smithsonian standard. 



The connecting the parts i and/ by rings and screws. Figs. 2, 3, 

 and 4, rather than by a single screw cut on the edge, is an improve- 

 ment, as the single wood-screw is apt, after a time, to adhere so firmly 

 that it is often difficult, and sometimes impossible with safety to the 

 parts, to separate it. 



It is not advisable to disturb the cistern unless it becomes difficult, 

 from the oxide of mercury which gradually forms, to make the ad- 

 justment of the mercury to the ivory point, as there is more or less 

 risk in doing so. Any one accustomed to such mechanical affairs, 

 with due attention to the plan, can, however, take out the mercury 

 from the cistern, refilter, clear the parts of adhering oxide, and re- 

 place them ; the instrument all the time being kept vertical, with the 

 cistern at top, as the mercury must not be allowed to come from the 

 tube. 



To insure a good vacuum by the complete expulsion of all air and 

 moisture, the boiling of the mercury in the tube is done in vacuo ; 

 and care should be taken to preserve it in good condition. 



To put up the barometer for observation, suspend the barometer by 

 the ring A in a good light, near to and at the left side of a window, 

 and, when practicable, in a room not liable to sudden variations of 

 temperature. Eecord the temperature, and then, by the screw 0, 

 lower the mercury in the cistern until the surface is in the same plane 

 with the extremity of the ivory point. As this extremity of the point 

 is the zero of the scale, it is necessary, at each observation, to perfect 

 this adjustment. It is perfect when the mercury just makes visible 

 contact. If the surface is lowered a little, it is below the point ; 

 and if raised a small amount, a distinct depression is seen around the 

 point. This depression is reduced to the least visible degree. A few 

 trials will show that this adjustment can always be made to a thou- 

 sandth of an inch. 



The adjustment effected, bring the lower edge of the vernier 0, Fig. 

 5 J by means of the milled head D, into the same plane with the con- 

 vex summit of the mercury in the tube. Looking through the open- 

 ing, with the eye on a level with the top of the mercury in the tube, 

 when the vernier tube is too low, the light is cut off; when too high, 

 the light is seen above the top of the mercury. It is right when the 

 light is just cut off from the summit, the edge making a tangent to 

 the curve. A piece of white paper placed behind, and also at the 

 cistern, will be found to give a more agreeable light by day, and is, 



