This section is from the book "The Gardener V1", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
There is at least no objection to your first suggestion: we have them hung in the centre, and like them so, because when they fall in at the top they let a stream of air up between the foliage and the glass. We are not aware that there is any boiler that does more work with the same amount of coal than boilers of the Cornish make, which, with the various improved saddles, we reckon the best. For very early forcing we recommend bottom-heat. The Gros Colman is quite equal to the Black Hamburg in the point you name. It is also a very robust and heavy cropping Vine, and takes the market remarkably well, on account of its huge berries and fine colour.