194- Instructions for the Scientific Expedition 



bourhood of the Lagullas Bank, will, of course, not be lost sight of 

 as the ships approach to and leave the Cape. 



The distribution of temperature over the globe must greatly depend 

 on the intensity which the solar rays possess on attaining the surface 

 of the earth after traversing the atmosphere in different latitudes. To 

 subject this point to direct inquiry in a mode which after many years' 

 trial has been found to give very satisfactory results, Actinometers are 

 provided, and accompanied with very precise directions for their 

 use. They should be observed only when the sky in the immediate 

 neighbourhood of the sun is perfectly free from visible cloud. On the 

 other hand, depression of temperature caused by diurnal and nocturnal 

 radiation by the only means we at present possess for that purpose, — 

 viz. that of thermometers blackened and exposed in reflectors to the 

 sky, — will form a useful and valuable supplement to the actinometric 

 researches. With a view to the collection of facts illustrative of the 

 distribution of temperature on land, wherever the ships may touch 

 with a prospect of remaining some days, no time should be lost, on 

 landing, in burying in the earth one or more bottles (filled with spi- 

 rits, if there should be danger of water freezing), well packed in 

 cases, or boxes stuffed with non-conducting matter, such as woollen 

 cloth, pounded charcoal, &c., but so as to leave easy access to the 

 neck, which should be wide enough to introduce the bulb and stem 

 of a good thermometer, so as to take the temperature of the con- 

 tained liquid rapidly, before it can have become altered by exposure 

 in the air on taking up the bottle. Bottles so arranged should be 

 buried at depths of three, six, nine, twelve, &c. feet*, according to 

 the facilities of penetrating the soil, and abandoned till the time of 

 departure, so as to ensure their acquiring the precise temperature of 

 the soil ; and when taken up should immediately have the tempera- 

 ture of the included liquids ascertained. In case of very prolonged 

 sojourn, monthly readings should be taken. The temperatures of all 

 springs and wells should also be diligently noted and registered. 



Connected with the transcalescence of the air, is the transpa- 

 rency of the sea. The stimulus of the solar light no doubt affects 

 the surface of mollusca at great depths, and numerous points of phy- 

 sical inquiry would be elucidated if we knew the co-efhcients of ex- 

 tinction of the solar rays by pure sea water. As far as the luminous 

 rays are concerned (or at least the chemical), the actual intensity of 

 these rays at various depths might be very easily ascertained, both 

 for direct sunshine and that of cloudy daylight, by the aid of Mr. 

 Talbot's sensitive paper ; which, duly guarded from wet by varnish 

 and interposition between glass plates, might be sunk, face upwards 

 in a small frame, while a portion of the same paper, cut from the 

 same sheet, should be similarly exposed on deck, and partially shaded, 

 inch by inch, from minute to minute, (or for a smaller interval accord- 

 ing to the sensitiveness of the paper) with a view to immediate com- 

 parison, between decks by a light not strong enough to alter the tint. 



A simple and convenient mode of photometric measurement is also 



* These are the depths adopted in Mr. Forbes's recent experiments. 



