134 MEASUREMENT OF BASE LINE, 
pansion was as before stated ‘007632 inches for each degree of 
the thermometer, so that it was easy to determine whether any 
excess or deficiency in the length of the tape agreed with the 
micrometer. The corrections from one to six degrees are as 
follow :— 
1° -00763 inches. 4° -03052 inches. 
Ze eOUSIG:  ,. 5°. Oe6ieee 
Be “02289... 6°. -04578 
-,One to eight revolutions of the micrometer represent the 
following fractional parts of an inch, viz.:— 
1 revolution. ‘005 inches. 5 revolutions. ‘025 inches. 
2 o ‘O10. 5 6 s 030: | gy 
3 a ‘OLS. 5: Ff es "OSB 1 we 
4° $2 a) een 8 ss 040 =, 
It will be seen by a comparison of these values that three 
revolutions of the micrometer nearly equal the correction for 
two degrees of temperature, whilst the expansion or contraction 
for five degrees of temperature nearly equals eight revolutions 
of the micrometer. The relative values of the micrometer 
of the clinical thermometer might be devised to answer the purpose. 
What is required is sensitiveness, combined with stability of construction. 
It will be understood that very delicate glass thermometers are so liable 
to break that they are practically worthless in field use. 
“The thermometers should be graduated to 0°2 of a degree, and should 
range from 32 to 125 degrees. I enclose a letter from Mr. Cassella, who 
appears to have given the subject some consideration. You might, at 
your convenience, arrange to see him and ascertain whether he has any- 
thing to suit my wants. Any expense will be met as soon as you 
advise me. 
“You will excuse my troubling youin this matter. I wish to have the 
most suitable class of instruments that can be procured, and I believe 
you can assist me better than anyone else. 
“T have the honor to be, Sir, 
« Your obedient servant, 
“(Signed) W. ALCOCK TULLY, 
“G. M. Wurprxe, Esq., “ Surveyor-General. 
«Superintendent Royal Observatory, 
“Kew, England.” 
