390 SCIENTIFIC RECORD FOR 1882. 



gen found the tension at 0^ = 0.015"", Eeguault, 0.02 ; at 20^, Hagen, 

 0.021, Eegnault, 0.037; at lOOo.Hagen, 0.61, Eegnault, 0.75; at200o, Ha- 

 gen, 16, Eegnault, 19.9"™. Though the values now found have no claim to 

 absolute accuracy (owing to the difficulty of taking readings with the 

 cathetometer through round glass), these exi)eriments at least make 

 certain that the Eegnault values for the vapor-tension of mercury, which 

 have passed into all text-boohs, are considerably too large. {Nature^ 

 XXVI, p. 168.) 



Professor Eucker, in a x)aper "On the calibration of mercurial ther- 

 mometers, byBessel's method," stated that the late Mr. Welsh, of Kew 

 Observatory, described to the British Association, in 1853, the methods 

 which he introduced of making and correcting mercurial thermometers. 

 The correction with which the author dealt was that due to the variations 

 in the bore of the tube. Mr. Welsh's method of making this correction, 

 which is still employed at Kew, is leso theoretically perfect than others, 

 and has been unfavorably criticised abroad. The author, in conjunction 

 with Professor Thorpe, has recently corrected a number of thermome- 

 ters with great care by Bessel's method. One set of three thermometers 

 was made for them at Kew and calibrated according to Welsh's method. 

 Afterwards the measurements necessary for the application of Bessel's 

 method were made by the Kew authorities, the calculations beiug per- 

 formed by the author and Professor Thorpe. The Kew thermometers 

 werethus subjected to the mostrigorous possible test, and they were able 

 to announce that in one instrument the errors left after the application 

 of Welsh's method were not greater than four-thousandths of a degree 

 centigrade, and in no case did they exceed one-hundredth of a degree. 

 As it is impossible to read on these thermometers less than a hundredth 

 of a degree with certamty, Welsh's method, as applied at Kew, is prac- 

 tically perfect. {Nature, September, 1881, xxvi. p. 467.) 



The alteration of the zero of thermometers after undergoing sudden 

 changes of temperature is a well-known i)henomenon, as is also the 

 gradual rise in the zero in thermometers during the first few months 

 after they have been made. M. Pernet has lately examined tbe ques- 

 tion whether the distance between the " boiling point" and the " freez- 

 ing point" of a thermometer is constant at all different stages of secular 

 alterations in volume of the bulbs, and finds that this is so, provided 

 the freezing point be determined immediately after the boiling point. 

 On the other hand, if the boiling point be determined and a long inter- 

 val elapse before the zero is determined, there is considerable error. 

 Sui^pose a thermometer to be (owing to recent heating or to long rest) 

 in any particular molecular state. In this state its reading will prob- 

 ably be in error ; but this amount (so far as due to the above cause) 

 may be ascertained by immediately plunging the thermometer into 

 ice, and observing the error of the zero reading. In order that a ther- 

 mometer should read rightly at any particular temperature it should be 

 exposed for a considerable time to the temperature for which exact 



