May 27, 1898.] 



SCIENCE. 



733 



umns and head B, by a thick screen. This 

 is very necessary. 



In order to get the best results from the 

 apparatus, many precautions are necessary. 

 After filling A and B with mercury, time 

 must be allowed for the compressed gas to 

 cool. The effect of changing barometric 

 pressure is nearly eliminated by so regu- 

 lating the quantity of mercury in F, that 

 its surface is in the small tube at the bot- 

 tom of the cistern, when the gauge is prop- 

 erly filled. Its area is then very small, as 

 compared with that in B and C. The 

 height of the meniscus in both tubes is 

 easily adjusted sensibly equal, by a little 

 manipulation. I always raise the mercury 

 above the point at which readings are to be 

 taken, and then lower it, so as to read on a 

 falling meniscus. This is highly important. 



Some trouble was occasionally experi- 

 enced at first, from electro- static induction 

 between the mercury in B, and the glass 

 above it. This was shown by distortion of 

 the miniscus when it was brought very 

 near the glass. The difiiculty was par- 

 tially, but not wholly remedied by putting 

 mercury in the outside open end of the 

 gauge head, and connecting it by a flexible 

 conductor with the mercury in the cistern 

 F. A complete remedy was effected by 

 moistening the inside of the gauge head 

 with a dilute solution of phosphorus pen- 

 toxide. This became completely dried by 

 the anhydrous phosphorus pentoxide in N, 

 but was, of course, not dehydrated, and 

 hence always remains conducting, and dis- 

 sipates the static charge. 



Large pressures, up to a thousand mil- 

 lionths or more, are readily measured with 

 this apparatus, by finding with the cathe- 

 tometer the distance between the mercury 

 in B, and the end of the head above it; 

 from this is quickly calculated the neces- 

 sary multiplier for the number of milli- 

 meters difference in height between the 

 columns B and C, also measured by the 



cathetometer, in order to express the result 

 in millionths. For very small pressures, 

 the micrometer wires are set at such a dis- 

 tance apart as to give a convenient con- 

 stant (usually 2), and the column in B is 

 adjusted this distance away from the glass, 

 careful allowance being made for the thick- 

 ness of the wires. Then the micrometer is 

 used for repeated measurements of the dif- 

 ference in height of the mercury in B and 

 C. The disturbing effect of bias is entirely 

 eliminated by giving the micrometer screw 

 a partial turn after each reading. Thus 

 the next measurement is made without any 

 knowledge of its difference from the pre- 

 ceding one, until the eye is removed from 

 the telescope. 



In my early experience with the appar- 

 atus, unusually careful measurements of 

 very small pressures were often made, to de- 

 termine how far its indications might be re- 

 lied upon. ■ In this connection I quote as fol- 

 lows from my notes, under date of February 

 20, 1895, concerning the last one of a series 

 of pressure determinations :" Following is 

 the last reading in detail, showing the ex- 

 treme accuracy of these measurements : 



" Mean of all the readings, .4346 M. 



" Eeadjusted zero point of micrometer be- 

 fore each reading of each set. Partially 

 emptied gauge and readjusted capillary 

 depression before each set of readings. 

 The first series has no known source of 

 error. The second and third series were 

 made during wind squall, and surface of 

 mercury was often tremulous. In the 



