40 COLLECTING DIATOMS. 



Special apparatus is required for collecting marine and pelagic forms. 

 My own is very simple, but in spite of its extreme simplicity it has, 

 nevertheless, proved hitherto sufficient in all my expeditions, and I do 

 not think that anything further is necessary. 



My apparatus consists essentially of a grapnel hook and a series of 

 nets (Frontispiece, figs. A E). 



The hook has three barbed ends, and is attached to a long cord. 

 This is used to secure or draw in any alga? which may be seen floating 

 about, or which may be found on the banks of rivers or shallow bottoms. 

 It is also trailed behind the boat, and so drags along the algoe and other 

 bodies which it happens to catch hold of at the bottom of the sea or 

 river. 



The nets are of four kinds : 



i. A bottom net or drag (Frontispiece, fig. B). The net is attached to 

 a heavy ring of tinned iron ; its weight is sufficient to make it scrape the 

 bottom, so that it collects the sand and other bodies over which it trails. 



2. A subsurface net (Frontispiece, fig. C). In this the ring of tinned 

 iron is rather lighter, and is, moreover, sustained by a piece of cork on 

 either side; the whole is balanced so as to be suspended at about ift. 

 to 2 ft. below the surface of the water. 



3. A surface net (Frontispiece, fig. D). The upper end of the net 

 consists of a wooden frame of square section, which is very long. This frame 

 as it is drawn along by the launch floats just at the surface, so as to 

 collect any diatoms which may happen to be there. This kind of net, as 

 a rule, gathers the most interesting kinds of diatoms. 



To the framework of each of these nets is attached a cylindrical bag 

 made of very stout cloth, to which again is attached the real net, which 

 should be made of silk gauze of the best quality, such as is used for 

 making fine silk strainers. 



4. An ordinary fishing net. This net is square and rather large, with 

 fairly close meshes, and is useful for collecting alga? and shells at the 

 bottom of the sea, which are so frequently covered with interesting 

 diatoms. 



All the nets, except the surface one, should have long stout cords 

 attached to them, so that they may work well and overcome a fair 

 amount of resistance. 



It need scarcely be pointed out that all gatherings should be made 

 against the current, and with the engines eased down to the minimum, 

 i.e., so as only just to counteract the current and so remain stationary, or, 



