90 Mr Dalton's Physiological Investigations on 



as the trap-rocks are concerned, they can be accurately named 

 in this manner only ; for we frequently find among them rocks 

 of the same composition, but differing in structure ; and, on the 

 contrary, their structure is often the same, when their composi- 

 tion is different. They are easily arranged in a few classes, by 

 their external characters ; and the class to which the rock be- 

 longs may be expressed in a single word, while its composition 

 may be denoted by two or three. There are some cases in 

 which we should still be obliged to use arbitrary terms ; but 

 this should not induce us to reject the only principle upon 

 which an expressive and convenient nomenclature can be formed 

 for the greater number of these rocks. I have long been in the 

 habit of using those compound terms in a district which presents 

 trap under a great variety of forms ; and I should mention the 

 mode that I have adopted, to render them as short and expres- 

 sive as possible, but that my observations and experiments on 

 the subject are far from being completed. 



Physiological Investigations arising from the Mechanical 

 Effects of Atmospherical Pressure on the Animal Frame. By 

 John Dalton, F. R. S. 



A. PERIOD of a century and a half has elapsed since the inven- 

 tions of the barometer and the air-pump. In this time the weight 

 of the atmosphere, its elasticity, its specific gravity, and many 

 other properties, have been ascertained experimentally with al- 

 most mathematical precision. The weight of the atmosphere, that 

 quality we have more particularly to consider in the present 

 essay, "is not constantly the same, as is proved by the rising and 

 falling of the barometer. It varies in this part of the earth from 

 ^ijth to ^Vth of the whole weight at certain times ; but those varia- 

 tions are gradual, so that it requires some days or weeks before 

 the weight passes from one extreme to the other. On an average 

 the weight or pressure of the atmosphere amounts to 14 J lb. on 

 each square inch of surface of the earth ; and, as fluids press 

 equally in all directions, every square inch of surface, whatever 

 may be its position, must be subject to the same pressure. The 

 surface of the human body, as well as that of animals in general, 



