Mr. J. Croll on the Physical Cause of Ocean-currents. 279 



subject, but rather to the defective methods which have hitherto 

 been employed in its investigation — that is, in not treating the 

 subject according to the rigid methods adopted in other depart- 

 ments of physics. What I most particularly allude to is the dis- 

 regard paid to the modern method of determining the amount 

 of effects in absolute measure. 



But let me not be misunderstood on this point. I by no 

 means suppose that the absolute quantity is the thing always 

 required for its own sake. It is in most cases required simply 

 as a means to an end; and very often that end is the knowledge 

 of the relative quantity. Take, for example, the Gulf-stream. 

 Suppose the question is asked, to what extent does the heat con- 

 veyed by that stream influence the climate of the North Atlantic? 

 In order to the proper answering of this question, the principal 

 thing required is to know what proportion the amount of heat 

 conveyed by the stream into the Atlantic bears to that received 

 from the sun by that area. We want the relative proportions of 

 these two quantities. But how are we to obtain them? The 

 only way we can obtain them is by determining first the absolute 

 quantity of each. We must first measure each before we can know 

 how much the one is greater than the other, or, in other words, 

 before we can know their relative proportions. In regard to the 

 absolute amount of heat received from the sun by a given area 

 at any latitude, we have the means of determining this with 

 tolerable accuracy. The same cannot be done with equal accu- 

 racy in regard to the amount of heat conveyed by the Gulf- 

 stream, because the volume and mean temperature of the stream 

 are not known with certainty. Nevertheless we have sufficient 

 data to enable us to fix upon such a maximum and minimum 

 value to these quantities as that every one will admit the truth 

 must lie somewhere between them. In order to give full justice 

 to those who maintain that the Gulf-stream exercises but little 

 influence on climate, and to put an end to all further objections as 

 to the uncertainty of my data, I have taken a minimum to which 

 none of them surely can reasonably object, viz. that the volume 

 of the stream is not over 230 cubic miles per day, and the heat 

 conveyed per pound of water not over 12 J units. Calculating 

 from these data, we find that the amount of heat carried into 

 the North Atlantic is equal to one eighth of all the heat received 

 from the sun by that area. There are, I presume, few who will 

 not admit that the actual proportion is much higher than this, 

 probably as high as 1 to 3, or 1 to 4. But who, without adopting 

 the method I have pursued, could ever have come to the conclu- 

 sion that the proportion was even 1 to 8 ? He might have 

 guessed it to be 1 to 100 or 1 to 1000, but he never would have 

 guessed it to be 1 to 8. Hence the reason why the great in- 



