368 THE POPULAR SCIENCE MONTHLY. 



likely to give correct indications, may really be in error tAvo or three 

 degrees Fahrenheit at some part of their scales. Thus the "fever" 

 thermometers in general use by physicians are almost invariably too 

 high in their readings. An analysis of the results of sixty-eight 

 thermometers of this description, verified in June of this year, will 

 show how great this error may come to be : one fifth had errors less 

 than 0*1 ; one fifth had errors less than 0"4 but more than 0*2 ; two 

 fifths had errors less than 0"7 but more than 0*4 ; one fifth had errors 

 less than 1'0 but more than 0*7 ; and occasionally a thermometer 

 was found which had errors exceeding 1 and more rarely one exceed- 

 ing 2. The thermometers on which the above deductions rest were 

 chosen to represent seven makers, and may be fairly taken to indicate 

 the liability to error in using fever-thermometei'S which have not been 

 compared w^ith authoritative standards. It is not unlikely that members 

 of the medical profession have been sometimes misled by the readings 

 of inaccurate thermometers, and they may have made such unfavorable 

 statements regarding the chances of recovery of patients "whose tem- 

 peratures were high, that the patient, under the influence of his 

 imagination, has given up the struggle for existence when a little more 

 hopeful view of the case might have imbued him with fresh courage 

 and led to ultimate recovery. 



The work at the Yale Observatory divides itself into t^vo parts 

 the establishing of the standard thermometers with wdiich thermom- 

 etors sent to the observatory are to be compared, and the -work of 

 comparing thermometers. The investigation of the standards them- 

 selves is by far the most tedious of the two ; and as the methods used 

 in studying the observatory standards are also the methods used, with 

 greater or less detail, in investigating the higher grades of thermome- 

 ters sent to the observatory, the methods will be briefly outlined. 



It will be necessary to recall some of the fundamental principles of 

 thermometry, however, in order to jiroperly comprehend the methods 

 of procedure in the case of standards : 



1. Glass mercurial thermometers slowly increase their freezing-point 

 readings as their age increases after the heating they undergo in filling 

 with mercury in their manufacture. 



2. The readings of the boiling-points are also increased, but in a 

 much less degree perhaps not more than one fifth as much as the 

 freezing-point. 



3. Whenever the thermometer is heated at all, the freezing-point is 

 lowered, and the amount of this depression is very nearly proportional 

 to the square of the temperature to which the thermometer is heated. 



4. It follows from 3, that if a thermometer is kept at the ordinary 

 temperatures, the freezing-point of w\ater will be indicated by a lower 

 scale reading than if the thermometer is kept at a low^ temperature. 

 Now, if we suppose the thermometer has been kept at the freezing- 

 point of water for a period of several days, and that the progressive 



