82 AtPARAtUS FOR l?ERfECTlNG THE 



bcfimilar to v^hat 1 have propofed. In addition i6v^hich, 

 the ingenious contrivance of Mr. Howard, publiflied in a 

 later number of your Journal, is alfo founded upon the fame 

 h)droftatical laws. But whatever may be the cafe in refpefi 

 to theory, no practical ufe feems yet to have been made of it, 

 owing moft probably to un fore feen difficulties in the execution. 

 The chief im- The principal of thcfe, in my opinion, as was mentioned 

 thf TxTricatTon" '" ^^^^ former memoir upoH this fubjea, is the extrication of 

 •fair which air from the water contained in the cooling veflel ; which air 

 ftopsthccurrenujjgjj-jggj^p^jjjgj^ partly by its own elafticity under a diminiflied 

 preflure, and partly by the heat, is accumulated in the upper 

 part of the cooler, and interrupts the two columns of tht 

 fyphon, fo as at laft to flop the motion entirely. For though 

 it may polfibly happen, by the negligence of workmen, that 

 the worm itfelf may not be fufficiently found to excludfe the 

 external air preffing into the cavity, and introducing itfelf 

 that way into the cooler, as is dated in the experiment of 

 I^Ir. Howard; yet I am perfuaded that in moft inftances, this 

 caufe may be taken for t life real one, inftead of the former, 

 which can never fail to aft in proportion to the perpendicular 

 height of the cooler, and the heat communicated by the worm.. 

 As it is impoffible to obferve by what aftual means the air is 

 produced, unlefs the whole apparatus were of glafs, fuch a 

 miftake may eafily be made. In fad, if metallic pipes could 

 not be made air-tight, even in a bended form, the exhauftions 

 in air-pumps and iiill more in great condenfations * would be 

 attended with almoft infuperable difficulties. 



P^or extrafting the air difengaged from the water itfelf, 

 during the adion of the fyphon, which in a few hours time 

 would neceflarily interrupt its motion, I have employed a few 

 hours of leifure confiruCiiHg the following means, which I 

 beg leave. Sir, to communicate to you; without pretending 

 to claim a greater portion of your attention thap th^ir remote 

 utility may appear to deferve. 



* In the experiments I made fome years ago, with an apparatus 

 conil:ru6ted/or conde/ifing different gafes and/or producing a Jlronger 

 artificial coldy than by any means employed before, I comprefled air in 

 brafs pipes more than to ^y of its original volume, which if we 

 fuppofe the cooler to be 15 feet high, is more than 50 times the dif- 

 ference between the external and internal preffiue of air in the 

 cooling worm of the new apparatus. 



On 



Not probable 

 that this air 

 •omes from 

 without. 



Plan for ex- 

 traftjng it. 



