64 Some Account of Dr. Others, 



quires a fenfible time of ten feconds or more to reftore the 

 equilibrium throughout the whole internal capacity. It is 

 this reftoratum that iticreafes or diminifhes the elafticity of 

 the air confined in the receiver, and thereby caufes the retro- 

 gradation of the mercurial column. Now I have found by 

 former experiments that a change of 50° in temperature ef- 

 fects a change of -/ c th nearly, in the capacity or bulk of air. 

 Tt follows therefore that in the cafe of reftoring the equili- 

 brium in condenfed air, about 50° of cold is produced; and 

 in letting in air to an exhaulled receiver, fomething more 

 than 50 of heat is produced. The fmall difference feems to 

 arife from this, that the condenfation of vapour in the former 

 cafe diminifhes the effect, and in the latter, if any there be, 

 increases the effect, that would arife from operating upon 

 purely dry air. 



The experiments and observations hitherto related go prin- 

 cipally to afcertain facts without any reference to the theory 

 of them : this, however, may be given in a few words, and 

 is the fame that is afcribed to Mr. Lambert by Meflrs. Sauf- 

 fure and Pictet, and by them adopted. He conceives that a 

 vacuum has its proper capacity for heat, the fame as air, or 

 any other fubftance ; and that the capacity of a vacuum for 

 heat is Iefs than that of an equal volume of atmofpherical 

 air; alfo that the denfer air is, the lefs is its capacity for 

 heat: upon thefe principles the phaenomena are eafily refer- 

 able to that clafs of chemical facts where heat and cold are 

 generated by the mixture of two different bodies. If this 

 theory be right, and I think there is little doubt of it, we 

 may hence be led into a train of experiments, by which the 

 abfolute capacity of a vacuum for heat may be determined; 

 and likewife the capacities of the different gafes for heat, by 

 a method wholly new : but this mult be left to future invef- 

 tigation. 



XIV. Some Account of Dr. Olbers, the celebrated 

 Ajlronomer. 



w 



ILLTAM OLBERS, M. D. and member of dif- 

 ferent learned focieties, was born on the nth of Octo- 

 ber 1758, at Arbergen, a village in the duchy of Bremen, 

 where his father was a clergyman. His father having been 

 promoted to a living in Bremen in 1760, young Olbers re- 

 ceived his education in that city; but in 1772 he loll his 

 father, who; befides being a man of great general learning, 

 was a good mathematician, and had a great attachment to 

 6 aftronomy. 



