220 PHILOSOPHICAL TRANSACTIONS. [aNNO 1777- 



coat was not in the least injured; it is therefore sometimes an original conforma- 

 tion, but apparently unnecessary, as the gut, we may presume, would perform 

 its office much more agreeably without it. 



XXXII. An Account of some Experiments made with an Air-pump on 

 Mr. Smeatons Principle; also some Experiments with a Common Air-pump. 

 By Mr. Edward Nairne, F. R. S. p. 6l4. .:, , , , ,. ;. 



As the following experiments were made principally to try the peformance of 

 Mr. Smeaton's pear-gage, it may be proper, says Mr. N., to observe the descrip- 

 tion of it, in his own words, in the Phiios. Trans., for the years 1751 and 1/52, 

 vol. 47, p. 420, or p. 247, vol. 10, of these abridgments. The pump with 

 which the chief of Mr. N.'s experiments were made, had the leather of its 

 piston soaked in oil and tallow, with oil in the barrel, and every precaution was 

 taken that no water should get into the working parts of the pump, except what 

 might arise in vapour from the substances under the receiver. In trying some 

 experiments with the pump having Mr. Smeaton's pear-gage, and with the 

 common long barometer-gage, the degrees of exhaustion as indicated by the two 

 were surprizingly different. By the common long gage it appeared that the 

 exhaustion was 300 times, whereas by the new or pear-gage, it appeared to be 

 6000 times. By which it appeared that the pressure on the surface of the 

 quicksilver in the cistern, and in the tube of the long barometer- gage, was 

 diminished to about a 300th part: the pear-gage being now pushed down till its 

 open end was immersed under the surface of the quicksilver in the cup, the air 

 was then let in, and the pump appeared by that gage to have exhausted all but a 

 6000th part of the air; or, in other words, the degree of exhaustion by this 

 appeared to be 6000 times. 



Finding always this disagreement between the pear-gage and the other gages, 

 Mr. N. tried a variety of experiments; but none of them appeared satisfactory, 

 till one day in April 1776, showing an experiment with one of these pumps to 

 Mr. Cavendish, Mr. Smeaton, and several other gentlemen of the r. s., when 

 the two gages differed some thousand times from one anotiier, Mr. Cavendish 

 accounted for it in the following manner. " It appeared," he said, " from some 

 experiments of his father's, Lord Charles Cavendish, that water, whenever the 

 pressure of the atmosphere on it is diminished to a certain degree, is immediately 

 turned into vapour, and is as immediately turned back into water on restoring 

 the pressure. This degree of pressure is different according to the heat of the 

 water; when the heat is 72° of Fahrenheit's scale, it turns into vapour as soon 

 as the pressure is no greater than that of three quarters of an inch of quick- 

 silver, or about a 40th part of the usual pressure of the atmosphere; but when 

 the heat is only 41°, the pressure must be reduced to that of a quarter of an 



