94 PROCEEDINGS OF THE AMERICAN ACADEMY 



in reaching and of measuring over 3,000 atmospheres. To my knowl- 

 edge Amagat has as yet given only a meagre account of his ma- 

 chine. Fortunately, however, Professor Tail * recently described the 

 " manometer a pistons libres " at length, in connection with his own 

 researches. Special comparison of the respective advantages of 

 Amagat's apparatus and the machine of the following papers seems 

 therefore uncalled for. 



The Bourdon gauge has been recently discussed by Mr. Worthing- 

 ton t and by Professor Greenhiil.J The high pressure Bourdon gauge 

 used in my work was of the kind described by the latter. 



Screw Compressor. 



2. Disposition. — The apparatus (Fig. 1) consists essentially of a 

 strong wrought iron barrel, ABC, the head of which, AA, is suitably 

 threaded, so that a steel screw, SSTT, can be forced into it. The 

 piezometer tube is attached at the end, CC, of the barrel. Barrel and 

 tubes are quite filled with oil. 



3. Special Devices for preventing Leakage. — The first of these is the 

 tinned screw. This is an ordinary well cut machine screw of iron or 

 steel, covered with a uniform thin adhesive layer of ordinary solder 

 by dipping it in a vessel of the fused metal. Soldering salts are used 

 in the ordinary way. When forced into their sockets, these screws 

 secure complete freedom from leakage, even at 2,000 atmospheres and 

 more. Gauges and other appurtenances may thus be attached to the 

 barrel, and removed from it with facility. 



The other device is the gasket of marine glue,§ or similar very 

 viscous liquid. A stuffing box is easily made, by which this sub- 

 stance is kept pressed against the threads of the screw, or against the 

 smooth surface of a cylindrical plunger. Now whereas the cement 

 admits of being shaped by pressure to fill up any cavity, it is yet far 

 too viscous to flow through capillary interstices, except after the lapse 

 of an enormous time (months). Thus I found the absolute viscosity 

 (5^/ cm. sec.) to be 200 X 10®, that is, about 20 billion times the vis- 



* Tait, Challenger Reports, 1873-76, Physics and Chemistry, Vol. II. See 

 Nature, Vol. XLI. p. 3G1, 1890. 



t Worthington, Nature, Vol. XLT. p. 296, 1890. 



\ Greenhill, Nature, Vol. XLI. p. 517, 1890. 



§ Supplied by M. Ducretet of Paris, the Societe' Ge'nevoise, and doubtless by 

 many others. It is a specially prepared mixture of rubber and shellac. To 

 thicken it, more shellac may be added. Perhaps pitch, or even molasses candy, 

 would be similarly serviceable. 



