428 TERRESTEIAL PHYSICS. 



bliugs) of the surface, wliicli are frequent ou Vesuvius, but wbicli are 

 very rarely observed. The apparatus being visited at least three or 

 four times daily, at the ordinary hours of observation, and the alarm- 

 clock announcing the moment when the shocks occur, the instrument 

 may always be put back into its normal condition. This is the reason 

 why I have not thought it necessary to add other parts, which would 

 render it more complicated. 



But for violent earthquakes, which compel the leaving of the house, 

 and which render the visiting of the apparatus dangerous, one can, by 

 the action itself of the violent shocks, register tbe character, the dura- 

 tion, and intensity of each of them, with the aid of an auxiliary appa- 

 ratus that I have long ago prepared in my mind, but which I have not 

 yet had executed. My principal object was to discover the slight move- 

 ments of the surface which hitherto escaped us entirely. The seismo- 

 graph above described has not only given me the surest indications of 

 approaching eruptions of Vesuvius, but it indicates also the violent 

 earthquakes which occur in Italy and the entire basin of the Mediterra- 

 nean, even the eruptions of Etna and of Santorin. 



In localities exposed to violent earthquakes this apparatus should be 

 placed on a solid foundation of masonry, constrncted immediately on 

 the surface, and protected by a covering of wood capable of resisting 

 the shocks. 



ON THE DISTRIBUTION OF FOREST-TREES IN MONTANA, IDAEO, AND WASH- 



OGIOxN. 



By W. W. Johnson. 



In accordance with the promise made, I submit the following views of 

 the distribution of forest-trees over that portion of the ISTorthwest com- 

 prising the Territories of Montana, Idaho, and Washington, or more ]iar- 

 ticularly the region extending from the forty-fifth to the fiftieth parallel 

 of north latitude and from the one hundred and tenth meridian to 

 the Pacific Ocean. This part of our continent is traversed by three 

 systems of mountains, the Eocky, Bitter Root, and Cascade Eanges. 

 The mountain system of the two first named occupy a si>ace of about 

 two hundred miles in width, or from about the one hundred and twelfth 

 to the one hundred and seventeenth meridian, while the latter lies in 

 the vicinity of the one hundred and twentieth meridian, and their general 

 direction is north and south ; these are approximate locations, and are 

 stated' as limits of description. The traveler through the Bocky and 

 Bitter Root Mountains would be unable to tell where he lett the one or 

 entered the other. The valleys of the main water-courses are broad and 

 timberless, while the banks of every stream are fringed with a belt of 

 Cottonwood, with willows, alders, and a small birch undergrowth. 

 While the mountain spurs and ridges v;hich form the boundaries of these 



