222 ANNUAL REPORT. 



lure and its allied sciences to go into an exhaustive discussion of the relations of 

 climate to Horticulture. The field is so broad and the connection between the two 

 is so complex and thej^ are so mutually interdependent on each other, that it is only 

 by a series of long continued observations and a careful study of the results outlined 

 therebj', that an intelligent understanding of this subject can be had. What we 

 shall have to say this evening will refer to the climatic influences of the United 

 States and more particularly of Minnesota and the Northwest on forest and plant 

 growth. But first we wish to refer to those metorological terms which by the oper- 

 ations of the signal service are brought so prominently before the public and which 

 it is necsssary 'that we should thoroughly understand before we can have an 

 intelligent conception of the meaning which they convey. Those who are interest- 

 ed in the weather (and in this capricious and variable climate it is a subject of per- 

 ennial interest) will often find in the predictions issued by the chief signal office, 

 references made to areas of "high pressure" and "low pressure," to a probable 

 increase or decrease of the sande. It will be found that this refers to the specific 

 gravity or weight, if you please, of the air, and changes in the same which from the 

 expansion produced by the heat of the sun, varying amounts of moisture, the rota- 

 tion of the earth on its axis and other causes is perpetually varying. Thus as an 

 equal amount of dry, cold air is heavier than an equal amount of warm air satur- 

 ated, or nearly so, with moisture and as the air within storm areas is always warmer 

 and contains a greater amount of moisture than outside the storm's influence, it fol- 

 lows that the cold, heavy, dry air rushes in from all sides to the centre of the storm, 

 but the revolution of the earth on its axis deflects these winds to the right of the 

 centre in the northern and to the left of the centre in the southern hemisphere, 

 causing the wind to blow in great spirals and thus setting up those great gyratorj' 

 sj'stems of winds, blowing around a calm centre, which are called cyclones, or areas 

 of "low pressure". An area of low pressure or cyclone is not necessarily accom- 

 panied by rain or snow. Sometimes, and often in Minnesota, it is the case that its 

 enerey is only expressed by high winds and cloudy weather, but it is safe to say 

 that rain or snow will be an accompaniment of the cyclone during some part of its 

 existence. In this latitude these great disturbances in the air move in a general 

 course from west to east and taking advantage of this law, it is possible to predict 

 the weather in advance of their occurrence; sometimes a product of the conditions to 

 which the cyclone give rise is the "Tornado" with the effects and appearance of 

 which, some of you are doubtless familiar, as Minnesota, though near their northern 

 limit is within the region of their occasional occurrence. As there is a general and 

 widespread misapprehension as to the proper use of the terms, "Cyclone" and 

 "Tornado," we append the following as explanatory thereof, from the Weather 

 Review of the chief signal office for October, 1885. 



"A cj'^clone is a large, gyratory storm, generall}' from 500 to 1000 miles or more in 

 diameter with a considerable area of low pressure in the interior. A tornado con- 

 sists of a very small and violent gyration of the air; generally much less than a 

 mile in diameter, with a rapidly ascending current of air in the centre, and low 

 atmospheric pressure very near the centre, although there is generally too much 

 violence of agitation for it to be observed, and it is specially marked by a charact- 

 eristic funnel-shaped cloud with a progressive movement." 



A peculiarity of the tornado is that it invariably occurs a little to the southeast 



