22-4 PHILOSOPHICAL TRANSACTIONS. [aNNO 1777. 



soaked in a mixture of water and spirit of wine was used to set the receiver on 

 the pump-plate, may have exhausted all but a lOOOth or even a 10,000th part 

 of the common air, according to the testimony of his pear-gage; yet so much 

 vapour must have arisen from the wet leather, that the contents of the receiver 

 could never be less than a 70th or 80th part of the density of the atmosphere : 

 yet it does not seem that any deficiency in the construction of Mr. Smeaton's 

 pump was the cause of his not being able to exhaust beyond the low degrees of 

 70 or 80. Had ne been aware of the bad effects of setting the receiver on 

 leather soaked in water and spirit of wine ; and had he made use of the precau- 

 tion to free all parts of his pump as much as possible from moisture, I make not 

 the least doubt but the air-pump, which he executed himself, would have ex- 

 hausted to as great a degree, as that pump has been seen to have done with 

 which the chief of these experiments were made. I must here again observe, 

 that if we only wish to know the quantity of permanent air remaining in the 

 receiver after it is as much exhausted as possible, it seems that it is by Mr. 

 Smeaton's gage only that we can know it. Again, when by the assistance of his 

 gage and the barometer-gage together, we have discovered that there is a vapour 

 which arises and occupies the place of the permanent air which is exhausted, it 

 seems that it is by the means of his gage only that we can discover what part of 

 the remaining contents of the receiver consist of this vapour, and what part of 

 permanent air. 



By some further experiments, and some of the former repeated, the results 

 were very irregular, sometimes the one gage showing the greater degree of ex- 

 haustion, and sometimes the other, in a very extraordinary and unaccountable 

 manner. For, during the course of these experiments on the air-pump it ap- 

 peared, by the testimony of the pear and barometer-gages, that the remaining 

 contents of a receiver, when exhausted as much as possible, was at different 

 times of different kinds ; sometimes it seemed to consist entirely of permanent 

 air, as when a little vitriolic acid, &c. was put in the receiver ; and sometimes 

 mostly of vapour arising from moisture, and but a very small proportion of per- 

 manent air, as when a bit of damp leather, &c. was in the receiver. 



XXX I II. On the Culture of Pine-apples. An extract of a Letter from fFil- 

 liam Bastard, Esq. of Kitley in Devonshire, p. 64g. 



In the front part of the hot-house, and indeed any where in the lowest parts 

 of it, the pine-apple plants will not thrive well in water. The way in which I 

 treat them is as follows : I place a shelf near the highest part of the back wall, 

 so that the pine-plants may stand without absolutely touching the glass, but as 

 near it as can be : on this shelf I place pans full of water, about 7 or 8 inches 

 deep ; and in these pans I put the pine-apple plants, growing in the same pots 



