APPENDIX. 195 



Q. 



As Dr. Halley's theory of winds is revived, and advocated with a good deal of 

 abihty, in Professor Mitchell's paper, referred to on pages 134 and .138, we will 

 point out some of what we conceive to be objections to it. 



1. As ajDplied to the trade-winds, it is entirely inadequate to produce the effects 

 observed. It is on the ocean that the trade-winds are most uniform, and most fully 

 developed. Let us see, then, what the effect would be, if the equatorial parts of 

 the earth were entirely covered with water. 



Suppose A B D C to be a section of one of the vortices of Dr. H., or Professor M. 

 (seen from the north side, and drawn in the form of an 

 oblong, instead of an ellipse, for convenience of calcula- 

 tion) , in which the lower current moves westward from C 

 toward D, and the upper eastward from B toward A ; and 

 let its horizontal length be 100 miles (which is, we pre- 

 sume, as much as they would desire, since the vortices are spoken of as being of 

 "moderate dimensions"), and its height two miles. 



Now, the extreme diurnal range of temperature on the surface of that part of 

 the ocean does not ordinarily exceed 1° F., and the difference between the two 

 extremities of the vortex could not amount to xoVir of 1°. Air expands about 

 :f|-Q of its bulk for each degree that its temperature is raised ; consequently, the 

 difference in the specific gravity of the columns at the ends of the vortex (A C and 

 B D) would hardly amount to j-g- oVo'o of the weight of either, or :f gg oV'o"o"o of ^^^^ 

 weight of the air in the entire circuit. But it is this difference only which consti- 

 tutes the moving force, while the quantity of matter to be moved is the air of the 

 whole circuit. Hence, according to well known principles in mechanical philosophy, 

 the velocity communicated is 4 s^-g eVo" o"o' of that with which a body would fall freely, 

 and is precisely the same as that of a body descending on an inclined plane, whose 

 height is to its length as 1 to 48960000. Such an inclination, amounting to no 

 moi'e than about -g^^- of an inch in a mile, would be insufficient to give the slightest 

 appreciable motion to a fluid placed upon it. 



Professor M. attempts to meet this objection by the following remark : " That it 

 (the cause in question) is adequate to the creation of a considerable wind, is proved 

 from the fact that it is upon this that the other, or permanent temperature, depends, 

 and that it is what determines the existence of two winds ; the land and sea-breezes 

 blowing in opposite directions every twenty-four hours." But neither of these facts 

 seems to be relevant. The tendency of water to resist sudden clianges in its tem- 

 perature, in no way interferes with the accumulation of heat in the equatorial 

 regions, and it is on this that the higher temperature of those parts depends. And 

 in regard to land and searbreezes, it must be borne in mind, that the diurnal change 

 of temperature on land, is at least thirty times greater than on water. 



2. We cannot understand how Halley's theory accounts for the westerly winds 

 that prevail beyond the limits of the trades. The following is the explanation, as 

 given by Professor M., after remarking that the explanation of the trade-winds 



