% BAROMF.TKR. 1 19 



its attraction does raise a small tidal wave in the atmosphere, 

 which is indicated by the barometer, but its effect is scarcely per- 

 ceptible. According to the calculations of Laplace, the joint ac- 

 tion of the sun and moon is only capable of producing a tropical 

 wind flowing westward at the rate of about four miles a day, and 

 the effect produced by the conjoint actions of Jupiter and Venus, 

 when nearest the earth, would be a very gentle breeze moving 

 about a foot in fourteen or fifteen days, or about a mile in twenty 

 years. 



The invisible and perfectly elastic fluid which surrounds the 

 earth is called the atmosphere, or atmospheric air. It appears to 

 consist principally of two distinct expansible fluids, mechanically 

 combined in different proportions, a single portion or atom of oxy- 

 gen gas being united to three parts by weight, or four by bulk of 

 nitrogen, with a very slight admixture of carbonic acid, perhaps 

 one-thousandth part of the whole. Air was formerly considered 

 as an elementary body, but the analysis of this rare medium is 

 one of the finest discoveries of chemistry. The atmosphere, al- 

 though apparently so rare and mobile, is nevertheless, capable of 

 presenting great resistance to any obstacles to which it may be 

 opposed. We shall see this more completely illustrated when we 

 describe the phenomena attending hurricanes and other windy 

 storms, but meanwhile it will answer our present purpose to simply 

 refer to its use as a natural agent in propelling vessels by means 

 of sails, and urging the sails of wind mills. Although it is, com- 

 paratively speaking, light, and in the ordinary acceptation with- 

 out weight, yet we must not forget that it is now clearly demon- 

 stratable that the atmosphere which invests our earth, presses 

 everywhere on its surface with a power of about 15 Ibs. to the 

 square inch. The famous Torricellian experiment proves this. 

 It is well known that if the mouth is applied at one end of a small 

 tube, the other end of which is immersed in water, that upon ex- 

 hausting the air from the tube by the process called suction, 

 the fluid rises swiftly and flows into the mouth; this is a philo- 

 sophical experiment, but well known to every child. Now, if a 

 person ignorant of the principle that caused the water to rise in 

 the tube, should be asked for an explanation, he might answer 



