REPORT ON THE PRESSURE ERRORS OF THE THERMOMETERS. 33 



From the above formula we find the heating effect of one ton pressure on water at 50°F. to be 

 nearly 



0"-16 F; 



and for each degree above or below 50° F. this number must be increased or diminished by about one- 

 tenth of its amount. 



This expression is very easy to recollect, and it gives the results with ample accuracy throughout 

 the whole range of temperatures (40° — 60° F.) within which my experiments were conducted. 



It is to be observed that Thomson's formula is strictly true for small pressures only. No 

 account has been taken of a possible lowering of the temperature of maximum density, or of a change 

 of expansibility, under pressure. Nor is it known how a considerable increase of pressure affects the 

 thermal capacity. 



Appexdix D. — The Apparatus employed. 



The plate appended shows in section and in elevation the Fraser gun in which the thermometers 

 and gauges were exposed to pressure. The following memorandum from the Royal Gun Factories 

 sufficiently explains the material, mode of construction, and dimensions of the instrument. The 

 plate is copied from the sketch which accompanied the memorandum. 



"Memorandum on the Construction of Testing Cylinder for Sir Wyville Thomson. 



" No. 1. Interior tube is made of mild steel, similar to that used for inner tubes of guns, and 

 containing a very small amount of carbon. The tube has been tempered in oil, and its limits of 

 elasticity when in tension are about 30 tons per square inch. The ultimate tenacity of the metal is 

 about 45 tons per square inch. 



" No. 2. The key and plug are made of similar material, and have also been tempered in oil, and 

 their limits of strength correspond to those of the inner tube. 



" No. 3. The intermediate coils B and B 1 were made by coiling a bar of this 

 section round a cast-iron mandril, and then welding this coil into a compact hollow 

 cylinder, open at both ends, and the fibre of the iron running round and round the 

 circumference of the cylinder. 



" The exterior coil C was made in a similar manner, on a larger mandril from a bar of this 

 section, and coiled in the reverse direction to the inner coil. 

 " The cylinders were then bored and turned. 



" The steel cylinder A was turned '01 of an inch larger than the interior of the 

 intermediate coils B and B 1 ; the coils were then heated to expand them, and were 

 put on to the steel cylinder and allowed to cool. When cold, the exterior of B and B 1 

 was turned - 02 of an inch larger than the interior of the cylinder C, which was then heated and 

 put on the cylinder B—B 1 . 



" The strength of the iron of which the coils are made is about 20 tons circumferentially, and 

 10 tons in a longitudinal direction with the cylinder. 



" No. 4 The inner steel cylinder A was subjected to hydraulic pressure before the outer coils 

 were shrunk upon it, of about 2\ tons per square inch, in order to test the general soundness of 



the metal. 



E 



