Temperature and Pressure in Balloon Ascents. 217 



or simply 



*(*-*) = J~ t HR-h) -»'(H-A')]. 

 V is readily deduced from this, and hence also ^. 



The tube No. 1 of the thermometric system is treated in just 

 the same way ; we shall get for it 



760 +V l + *t' 760 ~ l+ux 760 +V l+*t76Q' 

 whence 



\ + kx 



y V V l + «/' /' 



1 + ax y 



y is known by the experiment with the tube No. 1 of the barome- 



v 

 trie system, and ^ is determined in the apparatus in the same 



manner as we determined -^r. 



Thus x is deduced from the preceding equation. I am con- 

 vinced that in this manner temperatures and pressures would be 

 obtained with greater precision than by the direct observations 

 of the ordinary instruments. The aeronaut has only to attend 

 to the simultaneous closing of the stopcocks of both tubes of the 

 same number in the two systems; he does not influence the 

 results. 



It must be ascertained before starting that the tubes are ex- 

 hausted ; that is easy by means of the manometer, fig. 9. Glass 

 tubes may be employed instead of metal ; but their fragility is to 

 be feared. 



The thermometric tubes with a metallic envelope are easy to 

 construct, and I have used them frequently for low and for high 

 temperatures. A change of capacity in consequence of a variation 

 in the pressure is not to be feared : thus an air thermometer 

 exactly like those I propose for balloon ascents was surrounded 

 by melting ice; the levels of the mercury in the tubes of the 

 manometer were read off on the cathetometer. These levels did 

 not change when a complete vacuum was made in the cylinder 

 in which the ice was placed ; and yet the sides of the tube a b were 

 not a quarter of a millimetre thick. 



It might be feared that these thermometers are not sufficiently 

 sensitive to slow and continued variations of temperature; but 

 they are clearly more so than mercurial thermometers, however 

 small their reservoirs may be. I say, further, that they are as 

 sensitive as metallic thermometers, at any rate if these are not 

 formed of very thin, spiral, compensating plates; but in this 

 case these instruments are so mobile that they can only be used 



Phil. Mag. S. 4. Vol. 24. No. 160. Sept. 1862. Q 



