110 



' Birds also, and the smaller animals may be tlius preserved as spe- 

 cimens, without injury to their plumage or external appearance. 



For many of these purposes a very imperfect instrument is suf- 

 ficient, but it is in some instances requisite to abstract the air in a 

 very high degree from our vessels. This great power of exhaus- 

 tion has not yet been obtained without machinery, complicated, and 

 therefore liable to derangement, and the high price of these more 

 perfect instruments prevents many persons from availing themselves 

 of them. 



I have, on this account, ventured to lay before the Academy the 

 description of an air-pump, which I think will be found to unite 

 simplicity with great power of exhaustion, and which to all ap- 

 pearance is scarcely susceptible of being put out of order. 



In Cuthbertson's pump there are three valves, two of which are 

 opened mechanically, and of course are metallic, so that although in 

 theory it is capable of exhausting indefinitely, yet if we consider 

 how much the imperfections of workmanship, which are almost 

 unavoidable in every air-pump, are increased by the complicated 

 nature of his, and the excessive difficulty of its execution, it may 

 be questioued whether in practice it is of as much value as it 

 appears. 



Prince's rejection of the receiver-valve is a great improvement, but 

 the remainder of his apparatus is not equally elegant, and it may 

 be shewn tliat his valve-pump does not extend its power very much 

 independent of the complexity of such an addition. The instrument 

 which I now describe resembles his, in the mode by which the re- 

 ceiver communicates with the barrel, but the valve which com- 

 municates with the atmosphere is opened mechanically. The 

 apparatus for opening it cannot be attached to the piston, for 



