i 



Figure 8. — Hipp's registering aneroid barometer, with a telegraphic printer. 

 {USNM 2145441 Smithsonian photo 46/^0-D.) 



(fig. 12), but the other four utiUzed a physical prin- 

 ciple which had been proposed periodically for at 

 least a century — the unequal thermal expansion of a 

 bimetallic strip. This principle had been utilized by 

 watchmakers for a quite different purpose — the 

 temperature compensation of the watch pendulum — 

 but its possibilities as a thermoineter had been known 

 long before the mid-1 9th century.-" 



For the measurement of pressure, Secci, Wild, and 



2" In 1662 Hooke liad proposed the use of a bimetallic pen- 

 dulum for the temperature compensation of clocks. Thermom- 

 eters on this principle were described to the Royal Society 

 in 1748 and in 1760 {Philosophical Transactions of the Royal 

 Society of London, 1748, vol. 45, p. 128; 1760, vol. 51, p. 823). 

 Some systems used a bimetallic thermometer in the sun and a 

 mercurial instrument in the shade. 



Draper adopted, or rediscovered, the balance barom- 

 eter devised by Wren in the 17th century. In this 

 type of instrument (see figs. 13, 15) either the tube or 

 the reservoir of the barometer is attached to one arm 

 of a balance, the equilibrium of which is disturbed by 

 the movement of the inercury in the instrument.^* 



2' This instrument has been persistently associated with Sir 

 Samuel Morland (1625-1695). For example, A. Sprung of 

 the Deutsche Seewarte described his own balance-barometer 

 as a "Wagcbarograph nach Samuel Morland" (in L. Loewen- 

 herz, Bericht iiber die wissenschaftliclien Inslnmicnte auf der Berliner 

 Gewerbeaustelhng im Jahre 1819, Berlin, 1880, p. 230 ff). Sprat 

 {op. cit. footnote 10, p. 313) reported that Wren had proposed 

 "balances to shew the weight of the air by their spontaneous 

 inclination." This must, therefore, be Wren's invention, unless 

 he got it from Morland, who does not seem to have published 

 anything about the barometer but only to have described some 



108 



BULLETIN 228: CONTRIBUTIONS FROM THE MUSEUM OF HISTORY AND TECHNOLOGY 



