Page:Narrative of a Voyage around the World - 1843.djvu/31

 Rh the sea whenever the current runs from the northward or southward, should be continually observed, and marked in the log. A series of such observations would show under what circumstances the thermometer will indicate the effect of currents.

A minute examination of the tides, including all those data by which they may be accurately calculated, their local set, and the extent to which they are influenced by the periodic winds, and by the sea currents, arc so evident a part of your survey that it need not be dwelt on here. When practicable, their extreme height at the springs should be referred to a fixed object on the shore.

You will be furnished with a scale by which the force of the wind is to be expressed, and certain abbreviations by which the weather may be correctly described, and these are invariably to be employed in marking the logboard and log-books of both vessels.

The periods and limits of the trade winds, monsoons, and rains, will no doubt be a constant object of your study. It is true that your observations of them must be confined to the place where your vessels are; but still you will be able to collect a large number of accurate facts; you may perhaps pick up some authentic information from others; your journals of the Blossom and those of former navigators, will supply many connecting circumstances; and I feel confident that on your return home you will present to their Lordships the first consistent account of this interesting subject. Hitherto the practical seaman knows not where to seek for the periods of change, which are so essential to the due performance of his voyage; and those who would investigate and generalise the laws of these curious phenomena cannot find any distinct statements on which they can rely.