Some Practical Difficulties about Pendulums. 87 



This is easily proved by measuring the relative quantity and 

 heat of the heated air passing through the casing of the stove, 

 and that of the air passing through its furnace and up its 

 chimney. There is no stove that I have heard of to equal it. 



This theory of ventilation seems to give the result of pure air 

 with a minimum amount of fresh air introduced. By it a tenth 

 part of the quantity recommended by physiologists to produce 

 a healthy atmosphere will give air sufficiently pure to breathe 

 with comfort. It is not possible to heat by hot air in any way 

 except by heating from the ceiling downwards. No matter 

 how it is tried, the result will be the same. 



The next Paper was by P. F. Gulbransen, Esq., on 

 SOME PRACTICAL DIFFICULTIES ABOUT PENDU- 

 LUMS, AND HOW TO OVERCOME THEM. 

 The Subject was illustrated by a Barometrical Pendulum. 



With the advance of science has come the necessity of 

 dividing the day into constantly smaller intervals, thereby 

 causing greater difficulties in constructing clocks keeping record 

 of these small intervals. The best means we possess is the 

 seconds' pendulum. It is generally understood that the oscilla- 

 tions of a pendulum are always of the same duration, but in 

 practice there are formidable obstacles to be overcome. It is 

 impossible to surmount them perfectly, as they are caused by 

 the laws of nature, which govern all motions, and also by the 

 specific properties of the materials we employ. The only thing 

 to be done is to attempt to diminish the effect of these causes ; 



