204 



tering the water of the boiler, is let down through the engine to the 

 condenser, and there evolved. The '•' thermal agency" here is a 

 unit of heat let down from a body at the temperature of the water 

 in the boiler to another at the temperature of condensation, and the 

 " mechanical effect," therefore, cannot be determined, unless those 

 temperatures be given. Let us suppose then, in a particular en- 

 gine, that the water of the boiler is at 120°, and the condenser at 

 40°, during the working of the engine. The required mechanical 

 effect, calculated by adding the " sums" in the preceding table for 

 all the intervals fi-om 40° to 120° is found to be 328-6 foot-pounds. 



2. Theoretical Considerations on the Effect of Pressure in 

 lowering the Freezing-Point of Water. By James Thom- 

 son, Esq., jun., Glasgow. Communicated by Professor W. 

 Thomson. 



At the commencement of this paper the two following propositions 

 are laid down : 



I. That water at the freezing-point may be converted into ice by 

 a process solely mechanical, and yet without the final expenditure of 

 any mechanical work. 



II. That the freezing-point of water must become lower as the 

 pressure to which the water is subjected is increased. 



The first of these is given as being interesting in itself, and as 

 having been the original means of suggesting the second to the 

 author. It may be deduced directly by the application to the freez- 

 ing of water of the principle developed by Carnot, that no work is 

 given out when heat passes from one body to another without a fall 

 of temperature ; or rather by the application of the converse of this, 

 which, of course, equally holds good, — namely, that no work requires 

 to be expended to make heat pass fi'om one body to another at the 

 same temperature. The first being established, the reasonableness 

 of the second will readily be admitted ; because the ordinary suppo- 

 sition of the freezing-point being constant, would involve the ab- 

 surdity of a perpetual motion (or, more strictly, a perpetual source 

 of mechanical work) being possible. For if a quantity of water were 

 enclosed in a vessel with a moveable piston and frozen without the 

 expenditure of work, the motion of the piston consequent on the ex- 

 pansion being resisted by pressure, mechanical work would be given 

 out ; and there would be no expenditure of any thing whatever to 

 serve as an equivalent for this mechanical work given out, because 



