1G2 PNEUMATICS. 



work in it. It was supplied with fresh air by a 

 forcing pump. This was used with great, success 

 at Ilamsgate. Other contrivances have been used 

 for diving to small depths, which have answered 

 very well, such as strong cases for the body, to 

 keep off the pressure of the water, which were 

 supplied with fresh air by pipes from the surface. 

 A very good one of this kind is particularly de- 

 scribed in the Philosophical Magazine, vol. viii.p.59. 



OF AIR BALLOONS. 



The air-balloon is a machine, consisting of a bag 

 filled with air, so light, that it, together with the 

 bag, forms a mass which is specifically lighter than 

 the common air of the atmosphere. A cubic foot 

 of common air is found to weigh above 554 grains, 

 and to be expanded by every degree of heat 

 marked on Fahrenheit's thermometers, about l-50th 

 part of the whole. By heating a quantity of air, 

 therefore, to 200 degrees Fahr., you will just 

 double its bulk, when the thermometer stands at 

 54t in the open air, an,d in the same proportion you 

 will diminish its weight; and if such a quantity of 

 this hot air be inclosed in a bag, that the excess of 

 the weight of an equal bulk of common air, weighs 

 more than the bag with the air contained in it, 

 both the bag and the air will rise into the atmos- 

 phere, and continue to do so till they arrive at a 

 place where the external air is naturally so much 

 rarefied, that the weight becomes equal, and here 

 the whole will ■ at. 



The power by which hot air is impelled upwards, 

 may be shown by the following experiment. Roll 

 up a sheet of paper in a conical form, and by 

 thrusting a pin into it near the apex, prevent it 

 f'tom nriolling. Fasten it then by its apex, under 



