ON THE ANEMOMETERS OF PLYMOUTH. 243 



In Plate XXXIII. figs. 1, 2, 3, will be found graphical delineations of the 

 path of the wind at Plymouth for the years 1845 1, 1842, and IS-tS.aslaid oft" 

 from daily observations with this instrument: the scale is sufficiently large 

 to indicate the successive changes which have occurred in these years. 



On examining these annual types, we find, as in the former observations 

 published in vol. vii. of the Reports of the Association, that the general 

 course of the air is towards the land, and varies between N.E. and N.W. 

 The current appears to be interrupted in its general course by certain dis- 

 turbances or tourbillons which tie as it were so many knots in its path and 

 which are almost periodical. Thus in the type of IS^l, fig. I, there are four 

 of these disturbances, viz. about January, April, July, and November. The 

 general course of this type is nearly N.N.E. 



The type of 1842 presents also four marked disturbances, viz. about Ja- 

 nuary, March, August, and October. The general course here is N.N.W. 



The type of 1843 has again four tourbillons or knots, viz. January, April, 

 July, and October. The course here is a little to the east of north. 



These disturbances evidently approach a periodical form. Thus we have 

 a complete disturbance about the commencement of each year, which is 

 further shown in Plate XXXIII. fig. 4, in which are laid oft" the two months' 

 observations of the year 1844. About the months of March and April we 

 have a second disturbance ; a third about the middle of summer, and a fourth 

 about the period of autumn. These disturbances, although not perfectly co- 

 incident, still show so great an approximation, as to render their dependence 

 on a similar cause highly probable; and since they are found about the periods 

 of the vernal and autumnal equinoxes and of the solstices, it is not unrea- 

 sonable to conclude that they result from the peculiar position of the sun at 

 those periods. 



The complicated path or type of the wind at the time of these disturb- 

 ances is not unworthy of attention. In Plate XXXI V. fig. 5, will be found laid 

 off, on a large scale, the disturbance observed in the autumn of 1842. In this 

 example the wind is at the point ap, twice coincident in its general mean di- 

 rection, and has been twice thrown out of it, and there is a general resem- 

 blance in the course it has taken on both these occasions; in both cases it 

 has been traced southerly and westerly, as at a and b. The wind did not 

 recover its general mean course in this case until after October. 



These eff'ects point out the necessity of observing the instrument for a long 

 period of time, and not merely for the space of a month or two ; for should 

 the observations happen to be made at the time of these periodical disturb- 

 ances, and the results laid off on a large scale, very indefinite views would 

 arise relative to the general movement of the air. 



This method, adopted by Professor Whewell, of figuring the path of the 

 wind, although extremely ingenious and even indispensable where we wish 

 to present to the eye the successive changes which have occurred, is still 

 not the onl}' form under which the observations may be discussed. There 

 is yet another method of dealing with the register, which should be also 

 noticed. 



If we consider the different winds, when reduced to any given number of 

 points of the compass, as so many forces whose directions are given, and take 

 their rel.itive intensities for the time during which each wind is recorded, then 

 it is evident that we may proceed to deal with these forces in intensity and 

 direction in the same way as we should deal with any other number of mecha- 

 nical forces under similar circumstances, and hence obtain the resultant of 

 these forces representing the diff"erent winds for any given time ; and if we take 

 the intensity in terms of velocity, we may arrive at the mean annual move- 



r2 



